I'm going to ask my SIL to dress my niece up for a family photo
Why specifically her?
I want to take a family photo with my cousin and my niece. We're all very close.
Your sister in law should dress up however they want to dress up. If you don't want to include them because of that then that's on you.
But I want to make sure that my SIL is wearing something nice for the photo. I'm going to be very disappointed if everyone isn't looking their best.
The photo isn't only about what you want. The photo should be a memory for everyone that took part in it so it's wrong for you to only think about yourself.