I think it's because they saw her fight back against her cousin and beating him...because in all of the scenes we see in the past they just watch her do nothing and get beat up. That's the only thing I could come up as a difference in what happened in the past and present.
This is just a theory but...
There's a abuse going on in their house, not sure who is abusing who but they were really exited when they heard tia beat up their cousin, but would always ignore when he was bulling her, so they probably learn to ignore the abuse from home;hearing Tia defended herself cought their attention because they never thought of the possibility of self defense before.
Pretty sure the abuse is towards the mother and I'm quite curious why they cut ties the mother's side of the family if so.
Oooooh that sounds really plausible
Like I too am confused at the sudden turn of affection