Is it normal that my dad does this?

your father sounds ridiculous. he should always have your back first - you being his child. you have to realize that Child protective services usually come around and do audits of the foster kids' stay at the house to make sure there is no inappropriate activity going on. maybe your dad gets money for watching these girls, and if he makes them unhappy, they might tell the social worker, and the social worker will take the girls out of the home, and your dad will loose the money that he gets from the state for taking care of them ( if he gets money for taking care of them). this might be his motive for having their back instead of having you and your brother's backs. i dont know how old you are, but it sounds like you are in turmoil. make a plan, stick to it, move out and move on with your life. hope this helps.

It is somewhat normal.
Your Dad is basically taking you for granted, you are his special and real daughter, but he feels he must make allowances and be a little more sensitive to the other 'foster teenage girls', because basically, they could leave if told off!
You, on the other hand, are family. And therefore expected to put up with and make allowances yourself (still in his mind)

You need to tell him (or even show him your post here) that treating you this way is wrong. If anything your Dad SHOULD be treating you better than anyone, and 'expect' the foster children should understand. Of which they will.

Ask your Dad truthfully if you are still the single most important child in his life. Then tell him you don't feel like it. Ideally you should both spend some Dad daughter time together (only) on a regular basis (ie either an hour a day, or a whole day once a week, or both!)

maybe witn out a women he does not now how to talk or treat girls did he yell at u with ur mom maybe he is lonely

Sorry honey, it sounds like he's shagging them.