You can’t force sibling relationships between two stepchildren, and you can’t force a stepmom or a stepdad to be accepted into a parental role by the child they’re step-parenting. While it can be difficult for a parent when their child seemingly stubbornly refuses to accept their new partner and their children, it is so much harder for the kid to be gaslit into treating people who are functionally strangers as close and beloved family members. Emotional closeness between blended family members has to be formed over time, and there’s a good chance it won’t happen at all. 

It’s common for children in blended families to feel like their parents chose their partner over them, but in this case, the child felt like his dad chose his stepsister over him. After his dad married his stepmom, he started to feel like a stranger in his own family, so he turned to his paternal grandmother for support.