Everyone's already said it, but I'll say it again: You are not only NOT BAD, you need time for yourself to be the best mom you can be. You need alone time, time with no one tugging at your elbow to get them something from the kitchen, time with friends - to be whole. Alone time is like a safety valve that helps you let off steam so you don't combust around the kids.
I'll bet one of the things that makes this situation harder is that you probably leaned heavily on your sisters and family for support when the divorce came down. You were probably happy for their advice and consolation. Now, they're probably somewhat trained to think they should advise you all the time about everything! Oh well - time for the roles to shift a little. Sis is back on her feet, 30 years old, a GREAT mom (obviously) and the best judge of what will and won't hurt her four kiddos. Just thank your sisters profusely and sincerely for everything they've done for you to get you through the mess, ask them to suspend judgment since they can't really walk in your shoes, and tell them that if they can't, you hope they will be able to bite their tongues because you're gonna have to agree to disagree on this. Then start getting together a list of great sitters from friends ("friends" who refuse to share sitters aren't "friends," btw), so that you don't have to listen to your family caw at you if you use one of them to sit.
By the way, everyone does things differently, but here's the approach I took with my daughters when I was ready to start dating them. I sat them down and told them that grown-up mommies needed to have friends and play dates just like kids, and that I was going to do that sometimes. I told them that some of my friends would be guys, some would be girls. At first I decided not to introduce my kids to any of my dates "unless it got serious," but then they were intensely curious about where I was and it seemed to cause them anxiety that I was leaving and they didn't know anything. I realized I was making it too mysterious (and consequently more serious than it needed to be) by hiding stuff from them. So I took a deep breath and the advice of a friend: I started to introduced my children to my dates by saying, "this is David, one of mommy's friends I told you about. We are going to a movie." At one point, my older daughter asked me, "are you going to marry him?" about one of my dates, and I said, "no, sweetie. We are just pals, just like you and Jacob (a school pal). When I am ready to get married to someone, you will be the first person to know." They seemed to accept this very well. I ended up glad I took this approach, because I was single for a really long time, and it would have been horrible to try to hide dating for so many years!
Good luck with your family and have fun!