Hi!
Literally yesterday I decided to retry this scenario, as I never managed to get the negative sentiments box checked in the first try. And this time... it worked! I deleted the family from the first try and completely restarted the challenge.
I don't know if any of the changes I did helped, but these are some things that I did differently than the first try:
The first time I made the oldest sister fight with the brother (yes, I made them fight, because I like to tick off all the boxes 😄). It was really hard then to make them friends, but somehow I managed (even though the bug still didn't let me continue). The second time I made the younger sister fight with the brother, it was much easier to make them friends after the fight.
The first time it took me several real life days to make them all friends, and it wasn't my main focus, I had gotten them all jobs and everything. Second time I solely focused on healing the negative sentiments and it took me like 2 real life hours. I made them dance together and apologize until I actually saw that the sentiments disappeared.
So my guess is that either it is something that was fixed with the last update (because I tried a lot of other tips and nothing worked for me either) or you have to heal those sentiments in one sitting. Either that or it's just a bug that happens on random playthroughs.