My Dysfunctional Mother

My parents separated over a year ago. I am almost 21, married, expecting my first. I have two sisters who live with my mother, one who is 19 and the other is 13.  My mom got married at 19 and so I wonder if this is why she is now acting so stupid. She is seeing 3-4 different guys, and I mean seeing them sexually. My sisters see it first-hand so I am going off what they tell me. The 19-yr old (call her A) says my mom stays out late almost 5 nights a week until maybe midnight or so. I'm not crazy about that since my youngest sister (call her B)is still in school but whatever. I told sister A that it wasn't any of our business unless things got out of hand and were affecting sister B. I said so long as she is not bringing the guys around the house I will keep my nose out of things. Well, today sister A informed me that sister B walked out of her room and saw my mother sitting on her bed, butt naked, with a phone down by her crotch. The door wasn't even closed! Then sister A said she even walked into he kitchen one day and happened to look in my moms room to see her taking a picture with her pants half-way off. Now, I can take sister A seeing this pitiful mess because she is an adult and she knows better than to act that way, but I cannot stand to think of my 13-yr old sister seeing my mother do that. So I plan on talking to sister B Sunday about whether she wants to stay living with my mom or if she wants to move in with my dad. I'm just trying to decide how much, if anything at all, I should tell my dad if sister B decides she wants to live with him. And I would really like to get the ball rolling on that if she does. Please give advice, me and sister A are at a true loss of what to do.

EDIT:--- I'm not calling her dysfunctional because of her sex life, I am saying that because her relationships with my sisters and I are terrible but she somehow still thinks she hung the moon. I moved out 2 days after I graduated high school. I couldn't stand living with her before my parents divorced. She thinks I'm going to want her in the L&D room when I am having my baby, we'll I hate to say it, it would feel like having a coworker or someone along that lines in there. I just don't have a good relationship with her and she is hurting the relationship she might could salvage with my sisters.