*Bachelor Pad gets a Guest*

Ok, it's been way too long since i updated this diary. I live with my brother and my cousin. It's bachelor heaven. Ping bong, fusball, pool, booze, you name it . . .
Recently, (and i mean RECENTLY!) my cousin met the girl and they started dating. She's about 10 years his junior but she's really nice, cute, smart, blah blah blah. Anyway, she's moving in with us!! THEY'VE ONLY BEEN DATING 2 WEEKS!! I told him it didn't bother me 'cause he says he's going to move out and buy a condo with her in a month or two.
Here's my question. Should i slap him? Tell him to wake up and look around? I want it to work for him, i want him to be happy, but i can see dark clouds on the horizon. Young girls change so easily. They change their minds, get bored, you know the deal. What should i say? Should i let him play his hand and see how it plays out? Aaaagggghhhhh.