T.W.
Wow, I am worn out reading all the extras you are going to have to do in these replies, just to get through the day with these kids.
I would recommend a mom's helper. A middle school girl that lives in your neighborhood that you know or have talked to and have her come over at a set time to just play with the kids. Pay her $5 an hour to play with the kids inside or outside and you can have a little bit of time to make dinner, get places set, clothes laid out for bath time, etc. Have her also to help pick up the kids toys while you are bathing the kids.
Let me tell you, my daughter does this for a lot of mom's in our neighborhood and during the summer she works for all of them everyday. It's first come first served on the days they get and hours and then they are all on a schedule and she loves it. It gets her out of the house, keeps her busy and she makes a ton of money.
This would be the most help for you. Have your sister chip in on the money. Even if it's just a couple of hours 2-3 nights a week, you'll be amazed at what you can get done and how much help she is.
Then you don't have to make all these charts and crafts and such. Unless, you just have a bunch of time laying around to do those things. Kids have a very short attention span and don't want to be tied down to organized activities all of the time. They just want to play and do what they want.
Also, 4 year old boys and girls play totally differently, so having someone who can play games with them or chase or hide and seek would be something fun that they all can and would do together.
Good luck you'll do fine what ever you decide.