That makes life a little more difficult. It's not easy coming up with romantic type ideas with kids in tow LOL.
In that case I would offer to pay for a special meal out or some other treat. IMHO a night at the GF, however grand, might feel a little bit of a waste as they would have to share with the boys and after staying in a villa sharing a room might feel a little cramped.
Afew ideas that might work
1) A balloon trip over Disney might be a nice idea.
2) A cruise to see Epcot Illuminations show is fantastic fun, we did this on a replica old fashioned wooden speed boat (It was called the Breathless cruise) for my birthday and it was a great way to watch the fireworks and a real thrill ride. As it's a treat for them you have to tell them to make the boys sit in the back seats as this is where they get the wettest

. Don't want your SIL looking like she's had a bucket of water poured over her, then placed in a winf d tunnel, then dragged through a hedge. They also do pontoon boat rides that are cheaper, seat more people, but aren't as much fun. I couldn't find much info about their offererings ( poor website design Disney if you're lookingin

) so I would suggest you could call the recreation department ( numbers on the link following) and ask them if they still do it. Breathless would be my #1 choice it's a lot of fun, we had dinner at the Cape May Cafe before the boat trip. As a guess to cost you'd be looking at about $100 for Cape May and $200 for the cruise.
http://disneyworld.disney.go.com/wdw/moreMagic/otherRecDetail?id=OtherRecSpecialtyCruisesDetailPage. The link is misbehaving here's the number
CALL FOR RESERVATIONS(407)WDW-PLAY (939-7529)
3) Trip to Cirque , if the boys are not interested in Cirque, they could always be allowed to run riot in DQ for a few hours.