I too am still having this issue and have been having it for a long time. In the screenshots, I have my sim (grandparent) trying to plan a stay over on a Friday in the calendar. None of her family members outside of her household are available for the stay over. I can click on any one of the family members in the relationship panel to plan a stay over. But I still cannot then add the other family members as guests to the stay over. I was able to plan stay overs successfully with my desired sims on March 5, 2024. But at latest as of May 2, 2024, I was not able to plan stay overs with my family sims.
I am having this issue as well but it seems to be random. For one of my sims she could plan a stayover with her boyfriend from the relationships tab sometimes but not the phone. With another sim he hasn't been able to plan a stayover with his girlfriend at all. I end up getting another family member who's not in a romantic relationship with the sim to plan the stayover. I do have mods installed.
This is happening for me as well now, I can’t invite my sims parents or one sibling, nephews are fine but her siblings and parents are unable to be invited for a stay over
