Need advice how not to piss off SIL...

I need some help trying to figure out how to tell my sister in law I won't be babysitting for her when my baby is born. I'll be 12 weeks on Wednesday and it's been an extremely rough pregnancy so far and she had super easy pregnancies with all 3 of her kids so it's like She doesn't understand my pain.... She has a 10 month old a 7 yr old and a 14 yr old. My husband and I have have decided that I will not be babysitting when the baby is born as I am a first time mother plus her kids have no rules and can be very rude and don't listen so it is stressful. If I've had a rough night (up all night throwing up with no sleep) I'll tell her and she gets so pissed at me and still drops her kids off at my house... There are plenty of relatives that can baby sit on call at random so it's not like it's a huge deal or hard to find someone else on short notice. I need help figuring out how to tell her without pissing her off that I won't be babysitting when the baby gets here... Please anything will help! I'd love to keep helping her but my husband and I feel that since I'll be a first time mother I should be giving all my attention to our baby considering he doesn't get paid time off so he'll only be home for a week at most with me and the baby...I swear its like she think after the baby and I come home I'll be right back to babysitting :( to add to it she would rather me watch them at her house because she doesn't want to leave her house 5 minutes sooner than normal to drop them off at my house. But when I have to come over to her house my husband has to drop me off on his way to work...at 3am. Then I get to come to her house and sleep on the couch which is not comfortable...she's selfish to me.