have your brother tell you when your mom goes shopping for groceries or something.
then, you can pull the gorilla+banana idea on your dad while he's home alone, or pull the "walk around the house like nothing is going on" idea, or anything really....
then once your dad knows, have him call up your mom and be like "hey _____ (insert your name here) would like some ________ (insert beverage or food here), he's hungry (and/or) thirsty." and then just hang up.
or you could call yourself and do that.. just be like "hey mom can you pick up some OJ while you're there? thanks!" and then hang up.
first idea would be better cause she'd come racing home with no idea.... then you could even potentially play it off as your dad was pulling a joke on her.... meanwhile you are about to surprise her in a completely different way hahahaha
JOKES ON JOKES ON JOKES...... JOKECEPTION.