DON'T feel guilty. BUT prepare for a very hard week, as your girls will make it clear that they can't entertain themselves at all.
Actually, I think a week is probably too long, we find that long term bans (here we ban screen time - no TV/Wii/PC at all) don't work well.
We have had a couple of weeks vile behaviour from DS (an only child, 11), defiance, outright cheek, non-cooperation etc. etc.
One night last week, after the bedtime from hell, DS wouldn't turn lights out when asked, and did the "you can't make me" thing.
DH went up, got DS out of bed, made him stand up straight against the wall, and read the riot act.
He said that from now on all privileges would be earned, and privileges include almost everything except breathing!
If he wants breakfast in the morning, he gets up, showered and dressed without all the backchat. When he comes in from school, he must put his coat away (not just chuck it down the hall), do his jobs (homework, guitar practice, empty dishwasher ) before asking if he can watch TV/play out/play on computer. Otherwise the answer is automatically "no".
He will treat us with respect, and do as he is asked, or no meals/comics/treats.
other things were also pointed out (can't go thru it all).
This was all said firmly, calmly, but loudly, and with determination.
He was then told to get in to bed, and his light was turned out.
He was crying (which made us feel VERY guilty indeed).
BUT since then, he has been much better, and we are having much more fun as a family, and he is getting his TV & puter time. This morning we were dancing and laughing together before school !!!!!
I don't know if it will last (typing here with fingers crossed), but it's worked so far.