They should feel grateful for their amicable relationship and leave it at that.

You can’t force a sibling relationship 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, and that’s okay. A kid whose age is in double digits isn’t going to accept a new parent into their life as quickly as a toddler, and there’s nothing wrong with that. If two stepsiblings have a relationship that’s closer to that of a roommate than to that of siblings, you shouldn’t demand that they become more emotionally attached than they are. That’s just going to drive them further apart.