By Chazoras haShatz on Rosh Hashana, I noticed 1 family (mother + 3 daughters) standing up for every Beracha (and then sitting again) Is that a (forgotten) Halacha, or just nice to do? Thank you.
There is no obligation to do this. It appears that the family have taken it upon themselves, and this is fine, although there is no halachic source for the idea.
Note that some have the custom of standing up for the entire repetition of the Amidah, but this is not done by many for the very lengthy repetition of Rosh Hashanah.