When photographing siblings, you can't expect them to simply "look happy" together. A sibling relationship is much more complicated than that. My sister and I had a "you can come into my room, but don't stay too long" kind of love. All I wanted was to wear the jean jacket that was fashioned with her 80's collection of buttons. (too much to ask?)
When I pull my camera out for a sibling photo shoot, I know that there is no telling what may happen, but the best approach is to avoid asking them to act like something they are not. Photobooth style shots are the best way to warm them up and get them confortable in front of the lens and you are guaranteed to see their true personalities and relationship quickly be exposed.
Age-appropriate games are also a go-to when I try to see how close I can get the family. Even if they aren't the best friends that the parents hoped they were, they will pull together for some non-stuffy game of knock your brother down first.