I can't really blame them for keeping their distance even before he went off the deep end. Gaara's powers were unstable, and he was prone to unintentionally harming those around him before he got a handle on them. The fear 'n stigma associated with Jinchūriki, the grudge they likely bore for their mother's sacrifice, and the influence their mammoth dick of a father, the Kazekage, should've had on their treatment of Gaarahe seemed to be pretty cold to the kid even before he started sending assassins after him.
I mean, they could've been written in such a way that their family bonds overcame all of that. But I think that was the more reasonable course of action.