My wife is an only child but she has a cousing who's something of a younger sister to her. We took notice that recently her cousin and my half-brother started spending a lot of time together and they do arrive and leave together (although this might be on account of my brother having a driving license and the girl is still getting hers).
My wife thinks it'd be cute if they dated and it might tie out families closer and she would like to "nudg" them, I just say to wait and see if they make anything official and don't pressure the kids.