What is inappropriate enough to step in?

Sorry this is so long. My step-daughters sometimes push the envelope on what I think is appropriate. A lot of it might be just normal kid stuff. I was raised in a conservative Christian household by a mom who was sexually abused as a child, so we were very modest and careful with that stuff, so I don’t know if I’m overly sensitive about it. There is a lot of butt grabbing, twerking, flashing, long kisses on the mouth, *** grabbing and other physical things between the girls, ages 7 and 11 (she’s come out as a lesbian). They still shower together. They have made some comments about their mom’s house. It seems like it’s “watch whatever you want, do whatever you want, just don’t bother me.” They get left alone a lot or with their mom’s boyfriend. There are a lot of things that I hear them talk about. The 7 year-old said her mom’s boyfriend calls her bubble butt (which I think is weird) and she also revealed that the girls, mom, and the boyfriend all went skinny dipping one time. But once the 7 year old mentioned that, the 11 year old looked embarrassed and started minimizing it, and then so did the 7 year old, also talking about how the boyfriend could only look at their mom because they weren’t related. The mom acts like they are mature enough to handle adult things—for example, acrylic nails for the 7 yo or taking the 11 yo to watch House of 1000 Corpses recently, or when she was 9 telling her about personal issues with her and her boyfriend and told her about women who jammed hangers into themselves before abortions were legal. It also seems like we don’t know a lot of what goes on and the mom tells the kids not to tell us stuff. All that to say, I’m thinking of discussing some of this with my husband but I don’t just want to stress him out if there’s nothing to do about it. Outside of some things I’ve heard, the mom and her bf seem like decent, normal people. Husband said she struggled with being a mom and connecting to the girls. He’s also said, at times, that he takes what the girls say with a grain of salt. Anybody have thoughts they’d be willing to share here? I don’t want to seem like we’re untrusting of someone who is doing nothing wrong but if something ever happened to the girls that way I know from my mom it would affect their lives forever.