Had something similar in my life about 18mths ago.
My mum & dad, although not evil like your inlaws sound, made a conceous decision to treat me differently to my two brothers, and now my kids differently to their cousins. Basically, as I have managed to keep a stable family life without divorce or debt I am deemed to be not as requiring of their attention as my two brothers and their kids.
After 38yrs of it I flipped when I found they had made the 250mls each way trip to the town I lived in, looked after my brothers kids for a week and buggered off home without even calling us to let us know they were in the area.
I cut them off. Told them it wasn’t normal and I wanted nothing more to do with them. I am used to disappointment but my kids wont be dragged into it. Anyhowm we see them at family doos and are civil but I haven’t spoken to either my mum or dad in any other capacity since.
I have made it clear through brothers that if they wish to see their grankids it would be encouraged but I am not interested in having any relationship with them.
The last 18mths have been the best I have felt about my mother and father in 38yrs. Gone are the ‘I wish’ and ‘if only’. Now I know where I stand and its so much calmer.
My advice would be to forget about it. If they are so evil, trust me they will not change and will probably be talking about your unreasonable behaviour to justify their spite. Get on with things and if they want to make amends let them, otherwise move on