Agree that age may make a difference. Faber and Alfred both have adult books (or books for older beginners, both of which I have used. If there are young kids (5-6), Faber has a wonderful new method for 5 and 6 year olds. I have used it with a young boy and it has worked quite well with him. His older sister is in the new Alfred series, though I think I like Faber's piano adventures most - at least as a base. And of course, supplement widely. With some siblings, I will have them share some books, but not have them play the same pieces. For some, playing the same pieces can be motivating, for others it will stimulate a bit of rivalry. It really depends on the personality of each sibling.