It's likely you have an outdated mod that caused the bug with the relationships. What you're experiencing now sounds like further effects of that initial bug where the babies weren't related. Since the parents don't know the infants, they can't interact with them.
Some possible workarounds:
- Play as the infant and have them smile at the parent. See if this unlocks their relationship.
- Hire a nanny and try to get them to introduce the infants to one of the parents.
If this issue happens frequently, you should try the 50/50 method to test which of your mods causes the issue.
