I am trying not to giggle, because I remember how VERY, VERY SERIOUSLY I took myself at 22, but dude, seriously: get over yourself. Plenty of parents have kids with wide age gaps. And I think Abby's advice is poor: these will be your brother and sister. It doesn't have to be any more complicated than that.

Unless you have a different, unstated reason for not wanting to think of them as your siblings, but I'm trying to be generous.