first off, welcome to families!
Your family is taking advantage of you, and it's only going to get worse. Is your sister paying rent? If not I would discuss this option with your husband. You said that she cannot afford her own place, maybe you could still charge a smaller amount than what they would normally have to pay. Also you need to sit down with her and tell her that you are sick of having to do everything for her. If she werent living with you, she would have to cook, clean pay someone to look after her kids during the day etc. Tell her that unless she's going to start helping out, she will have to start paying you for baby sitting.
You say that you dont do anything proactive all day.... I'm sorry but you are raising three of your own children plus looking after your sisters youngest child. I think thats more than proactive. As for cleaning everyday- I think that picking up each day and cleaning on weekends is fine! Remember no one is perfect and those who are expecting you to be need to look at themselves in the mirror.
I also really think that you need to have some Mum Only time. It might be a day, or a couple of hours in the morning or afternoon or something. You need to get away from everything and just enjoy being you. Maybe you could start going for a walk, if you have one, take your dog with you. Tell your husband that you need to talk. Tell him how all of this is making you feel. Tell him that you miss spending time with him. No one can read minds, and if you dont tell people how you're feeling they arent going to be able to do anything to help. Your family probably dont realise that you're feeling this bad, so you need to tell them.
Oh and one more thing, your friend needs to realise that you have had 3 children, one of them recently and you do not have access to all the personal trainers etc that celebrity mums have so it may take time to get your figure back- please don't beat yourself up about it.
I really hope that things start to get better soon!

s