yes, waiting for the brother to heal...Think about it this way…if you didn’t (and are you also planning to spay the sister?) there’d be THAT many more kitties to take care of, and there are soooooooooo many kitties needing homes right now, everywhere. You are doing the right thing, don’t feel guilty. Slightly less cats in the world means more chance at quality of life for the already existing ones. This also means more happiness for the many birds and other wild species a feral cat preys upon.