Is it permissible for me to hold onto the zakat money designated for my disabled brother and spend from it on him myself when necessary, instead of giving it to my sisters due to a lack of trust in their handling of it?

If your disabled brother finds what suffices his essential needs, then he is not among the recipients of zakat. However, if what he finds does not cover all his needs, then he is among its recipients. Consequently, it is permissible to give him from it according to the Malikis and Shafi'is, unlike the Hanafis and Hanbalis who believe that providing for a brother is obligatory.