Playing tennis with Brother #3 as tennis is a two player game and the others are engaging in solo activities (if you exclude planting as a communal activity in a collective farming environment or drying clothes are a logical consequence of a tandem experience, not to mention "doing something" is a suitable answer with the clearly stated premise at the start of the riddle as a valid thing to 'do'). I've heard this one told as a tale of Five Brothers and sometimes they're locked in a room, limiting possibilities and actually including all new ones-
... sorry. I get long winded and by your brief statements and longer riddles I have to guess that this social interaction is boring you or. at worst, annoying.
What do a dead man, a cruise ship and emu have in common?