Question related to Step-People A quick background related to this topic.

My ex and I were married 8 years, with our child as a family for 5. My son is currently 8 years old. Two years ago we divorced. We share 50/50 legal and physical custody and our son goes to school in my area/district. I am heavily involved in every aspect of his life. My ex remarried very quickly a year or so after and moved in with her partner. My ex would like to erase me and have no involvement but I fought for everything very hard in court and spent a lot of money because she wanted to move far away and did not care about our relationship. So she really pushes the narrative of her new partner.

I received a Christmas Card that my son gave me (he does't really understand), and it was pictures of all of them but said "Merry Christmas" from the Johnsons (fake name). The name is my ex's new partner. My son does not share that name and has my last name as his mother did until she changed it. My ex really pushes that her new partner nieces and nephews are his cousins and pushed her new partner is his Mom (which let everything happen naturally).

My question is - How do I handle this? I want my son to live his life, my ex can be very manipulative, but don't want to live in a fantasy land of the truth. I only talk positive about my ex, and it has been hard to see her remarry so quickly but with this I want to also say the truth about everything in regards to cousins, mom etc. family. He has a very involved mom and dad and family. She is really pushing this on him. I could understand if one parent wasn't not around and the step-parent had to step up but it is not like that at all. Any help or feedback would be greatly appreciated.