I voted "other."
In general, I don't think kids "deserve" their own rooms or need to have their own rooms. In your SIL's situation, though, I can see that it's sort of tricky. Having the kids share rooms by sex (girls in one, boys in the other) doesn't seem like a great solution to me, unless these kids have a *particularly* close relationship already established (and, with their age differences, I imagine they don't). It would totally feel like the non-custodial kids were "guests" in your SIL's kids' rooms, in my opinion. Would they feel comfortable leaving anything there for the two weeks they were away between visits--knowing that it was in someone else's room? I wouldn't. The full-time resident of the room might resent having someone stay in there every couple of weeks, and the visiting child would likely pick up on this and feel uncomfortable. Not a good solution, in my opinion.
On the other hand, depending on their ages, I can see that it might not work well to have opposite-sex siblings sharing rooms, either.
I, personally, would feel like the non-custodial kids should have a space of their own (if we were able to provide it), but that it would be okay for that space to be shared--so maybe one bedroom for the two of them to share at our house. Then I would make the decision about your SIL's kids based on their ages, personalities, what they've grown accustomed to, et cetera.
Bottom line, I wouldn't risk bankruptcy to provide kids with their own rooms, but I would do everything in my creative power to help provide a little personal space for everyone and to avoid making my step-kids feel as though they were any less a part of our family because of their non-residential status.
Depending on their ages, I would also involve the kids in this decision--ask them what was really important to them, explain what our limitations were and let them help come up with solutions. Perhaps they would suggest, for example, the girls sharing one room and the boys sharing the other--in which case I think it would be an entirely different issue and might work out just fine.