Would you lie?

I have a 4 year old nephew that I absolutely ADORE but he has never heard the word no from his parents and therefore he tends to be naughty around other people. I usually babysit him a few times every week but just don't know if I can continue to chase the little one around. I'm 31 weeks pregnant and I have fibromyalgia, arthritis, a bad back, both knees need surgery, and my right hip has been messed up and caused extreme pain for years and I just don't know that I can deal with chasing a 3 year old around for hours because every time he doesn't listen or misbehaves he runs away to do more naughty stuff. My brother only has me as his babysitter and gets pissed if I say no for any reason. I know being on bedrest is not for good reasons but I'm honestly considering telling him I'm on bedrest after my next appointment at 32 weeks just because I can't handle the constant running around and stress of my wonderful high energy nephew. Last time I babysat him I physically couldn't walk by the end of the day because my back wouldn't support me. I'd like to just tell him no I can't watch his son anymore for a while but if I do that then he'll just tell me I'm an awful sister and that I'm going to be a bad parent and that I'm lazy because that's what happened last time I coudlnt watch his kid. If you were in my situation would you lie? Am I an awful person if I do?