For background I have been with my DP since my son was 2. He is now 7 and we also have a 2 year old daughter together. He is my son's main father figure, he only sees his biological dad around once a month and during the holidays. My DP and son are very close as a result of this.
Since our 2 year old daughter was born I have noticed that she treats the children differently. For example, our 2 year old gets lots of presents for Christmas/Birthdays whilst my son only gets one or very little in comparison. It's clear that our daughter gets a lot more money spent on her. She also offers to take our daughter on days out to the zoo, park etc with her other biological grandchildren but she never invites our son.
I understand that he is not her biological grandson, but my son adores DP and we are a family. As far as I am concerned when he chose to become my partner, he also took on my son as his own (which he has never disputed, as I said they have a brilliant relationship). AIBU to think that MIL should treat both of 'our' children the same? I am worried that as my son gets older he will see the preferential treatment his sister gets.