Let it go -- your life will be ultimately easier and more enjoyable if you do.

My MIL is very biased towards my SIL's kids over my kids in many obvious ways. So much so that my kids (who are usually oblivious to things like that), have noticed and asked about things on several occasions. I tell them I don't know and they'll have to ask grandma. It used to bother me a lot, but I've learned to just take a deep breath and let it go.

FWIW, we live close to MIL & FIL so we see them a few times a month, SIL lives 600 miles away. My kids are boys, ages 6 & 9, SIL's kids are girls ages newborn and 3.