Sisters can work if they're from the same House, or if there are reasons that they have different family houses. Abilities are often inherited through the family line, but that's not always the case :D
Cousins might work better for Hearts a little apart (like 4 and 6) and it'd be unlikely for some Houses to be related (1 and 10) Although with a good backstory, everything is game. A 1-10 Heart family relation could be pretty dramafilled ;D