Am I being selfish?

For 6 years, since my SIL had her first child, we've always said yes to babysitting (for free because she's family), mostly because we don't have kids and we like to help. In the last 5 years, she's had 2 more kids, one is 4 yrs, the other was born in February.
 Anyway, like I said, we like to help, but there are a few things:
Her kids are very rambunctious, like they don't listen well, don't go to bed until like
 1 AM, punch/kick and don't stop when asked, etc. (they can also be very sweet, don't get me wrong.. I love my nieces and nephews)
When she asks us to babysit, she usually starts with "hey what are you up to?" & when we say nothing, she asks.. So we don't have an excuse. 
She gives us a time frame when asked, but on the day of, she'll ask us to come a half hour or hour earlier, while also expecting us to stay way later than asked. I don't mind on the weekends, but she usually asks us for week days.. My husband works from 6am to 6pm most days, and I work 8-6 most days, so this is why it bugs us.. 
She doesn't let us go anywhere, not even in the back yard. We are trapped in the house. 
So my question is, is it wrong that she asked us to babysit through text and neither of us has responded yet? I'm 20 weeks pregnant, my husband and I work so much and are pooped by the end of the night, esp. now.. And I feel extremely guilty, because I know she is busy with 3 kids and I'll be in the same boat soon of trying to find babysitters, but this time I just want to say no.