This sibling photo is just kind of a funny random one at grandma's - Liam is dead pan and Jack is cracking up! Also note poor Jack's rolled up shorts as he got a boo-boo on the back of his knee.
Funny moments like this can't be staged or foreseen. Sometimes it's about what you just happen to catch. Have your camera out and ready even when nothing in particular is happening and then wait and see what unfolds.