AITA for undermining my ex and continuing to let his parents favour our son over his half-sister?

My ex cheated on me while I was pregnant with our son. He only told me because he got his now wife pregnant and had no choice. He wanted us to work things out but I knew I couldn’t play stepmom to his love child so I chose to divorce him for my own sanity.

Our families were very close. My late dad was his dad’s best friend and I grew up considering them like second parents. We’ve remained close even after the divorce.

They spoil my son a lot. They constantly buy him gifts, they pay for his school and all of his activities, they pay so that I can take him on vacation every year, etc. They don’t do these things for my ex’s daughter and it’s causing jealousy between the kids.

My ex banned them from buying our son gifts or paying for his school as they were blatantly favouring our son. So, his parents called me and asked if they could see my son when he was with me to give him his gifts and if I would let them continue to pay for his school., which I agreed to.

My ex and his wife found out and are now pissed at me. Apparently, my son was rubbing it in their daughter’s face that their grandparents love him more. They want the three of us to figure out a way forward but I don’t see how this is my problem.

AITA?