What happens in Vegas with Joan Rivers and daughter Melissa Rivers certainly isn’t staying here when the mother-daughter duo of drama head for the Strip to celebrate their assistant’s 20th wedding anniversary. It’s the bachelorette party that Sabrina never had, and the wanton weekend is one for the books — and the TV cameras of the WE reality TV series “Joan Knows Best.”

The ladies really let loose in Las Vegas when Joan was headlining at the Venetian, and we’ll find out what hi-jinx they got into Tuesday night on the season finale of the dueling divas’ Season 2. It’s “smotherhood” with the talented twosome living, working and playing together. No wonder those feuds erupt!

The 10-episode season has been tumultuous, to say the least, and Joan and Melissa’s antics have been explosive, exciting, eccentric and endearing. We have seen Joan embark on legal marijuana, Melissa deal with her ex-live-in boyfriend’s infidelities and Joan celebrating her 78th birthday with tattoos and fighting a possible sexual harassment suit. It’s been an escapade of sex, drugs and a bar mitzvah. They show class, confidence and a crate-load of chutzpah.

Mom and daughter are under the same roof now that Joan has moved from her New York apartment into Melissa’s Hollywood home. You just know they’ll keep invading each other’s turf and privacy until whatever sense of order existed becomes disorder. Somehow while filming “Joan Knows Best,” they also appear on “Fashion Police,” where Melissa as executive producer is Joan’s boss.

Melissa and I spent 16 days in the Australian rain forest for ABC’s “I’m a Celebrity, Get Me Out of Here.” It was therefore easy for me to ask, “Does mother know best? Does she get on your nerves on occasion?”

“I hope she’s reading this because, yes, absolutely,” Melissa laughed. “She really is living with me four days a week, and I’m her executive producer on ‘Fashion Police.’ We work together, and we live together. I think all mothers get on their children’s nerves because they really do believe that they know what is best for their children. She might be right some of the time, but not all of the time. She would beg to differ!”

We posted a report about the Las Vegas episode last Thursday, and here’s my full conversation with Melissa.

Robin Leach: Melissa, you see your mom differently than everybody else does, but what do you put her professional, personal and physical longevity down to?

Melissa Rivers: Professionally because she stays relevant. She talks about what we’re all talking about or what we’d all like to talk about. She never throws herself in time, she’s right on the cutting edge of what people are discussing. Physically, she is just one of those people who are just a go-er. And again personally, she is one of those people who are just a go-er. Even at this age, she has more energy than my 11-year-old Cooper. He’ll pass out before she does. We’ll have a full day, and she’ll be, like, OK, let’s go out to dinner, and he looks at her and says, “Grandma, can we please eat at home?”

R.L.: With mom living so close to you …

M.R.: You mean in my guest room? Let’s just put it this way, at least she’s not actually in my bedroom. I think she would be if she could if I wasn’t careful. She’s already taken over every place else. I live on a hillside, so the guest room is on the lowest floor, and then there’s the main floor, and my bedroom is upstairs. She has already taken two of the three and is working her way up the stairs.

R.L.: Is she cramping your … style?

M.R.: Oh, yes, but what style? I work and I’m a single parent. There is not a lot of room for having any style to cramp. She is very respectful of, well, let me think about that, you know I do, I live my life, and she’s as respectful as one could imagine her possibly being. Robin, are you asking if her proximity makes dating a little difficult?

(Melissa has been linked to Steven Hirsch, the founder of Vivid Entertainment, which has more AVN Awards than any other adult studio and was the first to introduce celebrity sex tapes, including those of Pamela Anderson and Kim Kardashian.)

Well, again we are just dating. When you are a single parent and dating, it only leaves room on those non-custodial days, with your child, because I have shared custody, and I like to think of it as I see him on my non-custodial days of my mother, as well. I’m dating — it’s not a relationship.

R.L.: Have you taken any flack because of his adult movies?

M.R.: I really do just sort of my live my life, and if I just started worrying now what people think, it would just be too overwhelming. After going through such a public breakup with my marriage and then a live-in boyfriend, I think I have such boundaries in my world that I don’t let anyone else in with their views. Just give me some credit for making the decisions that are right for me.

R.L.: Do you sometimes flinch at just how candid you are on this show?

M.R.: Absolutely, but if you are going to do a reality show, it’s got to be real. That’s one of the differences with our shows and a lot of the other shows on the air where we don’t try and hide the wart. We are not holding ourselves out there to be perfect. That was our stipulation for doing it.

We’ve been offered a million reality shows along the way, and they always were, like, “You guys will live in the same house, and, Joan, you’ll pick out dates for Melissa, and, Melissa, you’ll pick out dates for Joan, and hilarity will ensue.” It just didn’t work for us. I think the fact that we allow ourselves to be seen in uncomfortable situations that are very real is what makes it compelling.

R.L.: It may be a little bizarre to me, but is it also a catharsis that you can be so open and then nothing has to be buried under the carpet?

M.R.: In a way, I guess it is cathartic, but my mom was pregnant with me on “The Ed Sullivan Show,” so my life has always been fodder for my mother, and her life has always been fodder for her comedy.

Obviously, and especially being a parent, there are things that I do say we are not going there, but they really only pertain to my decisions as a parent and not about myself, plus, certain limitations that I place on what we can see of Cooper, but that’s the only place where I set those boundaries.

R.L.: What was the toughest segment for you on this year’s series?

M.R.: The fact that my live-in relationship blew up while we were in production and in real time and having to make the decision to let the cameras stay. Where I didn’t allow them to go is when Cooper was dealing with it and me talking to Cooper about it. That is where I made that decision. We weren’t married, but we were darn close. I’m great now coming out of it. I learned a lot. I try to be a rock. I have to be at this point of my life.

R.L.: Have you inherited some of your mother’s characteristics?

M.R.: Yes, that, and my father’s. People don’t realize I am very much like my father, which is why I think my relationship with my mother is so strong.

R.L.: Do you think that Mom will ever get married again? Would she like to, would she like to settle down rather than live with her daughter?

M.R.: I think she’d like to get married, but only be married when it’s convenient with her, so who knows. Yeah, I mean if it was the right situation, she would get married.

R.L.: And you?

M.R.: Who knows? I kind of have a little bit of your feelings about marriage. I’ve been there, and I’ve done it, and, boy, was it expensive. The important thing is that Cooper is great, and we are thrilled with our show and so excited that our finale this year is taking place in Vegas, and everything is good.

The “What Happens in Vegas” finale of “Joan Knows Best” airs Tuesday on WE.