M.P.
I am a retired police officer. What you describe indicates the possibility of something being very wrong. It is extremely inappropriate for an adult man to have pictures of teens on his site. And the pictures would indicate the possibility of a lesbian relationship.
Teens do sometimes explore various approaches to sex. What the other girls are doing may just be exploration but to post the pictures on the Internet doesn't sound right. It has been my experience that teens who are exploring are unsure and so wouldn't be so blatant about it. Perhaps the pictures are a form of acting out their rebellion. This form of acting out is also not appropriate.
I would not permit my daughter to be at their house. I would talk with the mother and see if she knows about the pictures; both those of the girls and of her husband. I would ask her what the rules are at her house and how she enforces them. I would ask why the other girl is living with them. I would ask for the name and phone # of the other girl's parent(s). I also might talk with the Juvenile Division or Sex Crimes Division of the local police.
And most importantly I would talk to my daughter in a calm straigt forward manner about my concerns being careful to not be judgemental. Perhaps start by asking her what she thinks about this and really listen. Then tell her about your concern. Ask her who took the pictures. My main focus would be on the father's site and his pictures of teens and how this shows a highly probably possibility that he has a sexual interest in teens. Emphasize that you love her and it is your responsibility to protect her.
Having the girls at your house gives you an opportunity to observe them and become more aware of their personalities and interactions with each other and your daughter. I wouldn't quiz the other girls. Actually, I would try not to quiz your daughter. My daughter would clam up when I started what she called "20 questions." I knew enough to make me anxious but didn't really know what was going on until she was grown. Fortunately she has grown up to be a responsible adult and parent. While our kids are teens it's easy to feel like they are ruining their lives. I found it much easier to communicate with my daughter when I reminded myself that I needed to assume that she would use good judgement and would be more apt to allow me to participate in knowing about her life if I could remain calm and expect that she would work it out in an OK manner. Probably not the manner in which I would prefer but it would be OK. And it has turned out to be that way.
I just noticed that you have 2 teen daughters. Is the sister older or younger than the 15 yo. Will she talk with you about what is happening? She may be aware of things that she would like to talk about but won't bring up herself.
My thoughts and prayers are with you. Raising teens is very difficult for most of us.