I also had to do this with my inlaws.
It was bad enough they gave me a hard time, about all sorts of things.
Then to watch them treat my husband (their son) so differently and favor his sister was heartbreaking.
But when they mostly ignored our amazing daughter, her accomplishments and told me I should not allow her to go away to college (an amazing perfect college that SHE loved).. And yet attended every event their grandsons (Their daughters sons) participated in..
I was done. They said they would not attend daughters graduation OR her party, because I had never allowed them to be a part of her life or her graduation party..
I was furious. If anything, I am known for my annual school calendar that I sent the family each school year with a monthly update.. I also highlighted the important must see events. My gosh, I was PTA President at 2 of her schools, you can imagine..
The way I handled it was by mailing them a letter. I included specific instances, I had emails from all of the years our daughter had been in school starting from Kinder.. I was able to send them many of these.. showing that they had been sent and they had replied.
My husband had even gone to a therapy session with his mother when our daughter was born and my husband noticed the difference in her behaviors towards us vs.. her daughter.. Nothing changed.
I now encourage my husband and daughter to go and visit. I have to force them sometimes..
But I have not once wondered, "Gee I wonder how MIL is doing".. Good riddance.. Their loss. Instead I am surrounded by the people I love and enjoy and that also love me and my family the same way..