She's not actually with anyone, she's temporarily living with someone until she gets her own place. Take that however you will, Garen.

Generally, my thought too on making the "my sister's been missing for two months" thing work, is that perhaps he was on an expedition with some of the rangers and out of Union for a few months. This would explain it. Handwaving the "well why didn't you call" excuse, of course, because no time.