My mom wants me to babysit

Ok so I’m 23 with my own family now but my mom just recently called me asking if I could babysit her baby that’s 3 weeks younger than my baby boy (4 months in 4 days) while she works in the mornings. And I’m a first time mom also. I don’t think she is going to pay me for it and we live around 20 something minutes apart and we aren’t doing so amazing financially and also my boyfriend does not like my mom and vise versa and I know this will cause problems with us. And I mean I could consider if she did pay me but I don’t think she was going too suggest it cause I mean if she is going to work is to get extra money in the house not to pay me. Plus when I was still living with my mom every time I could change a diaper and feed the baby (my siblings) she would start work and she wouldn’t change a diaper or feed the baby no more it was considered my job now and plus cleaning the house at the same time. But not paid for I just had to do it and it makes me mad cause I mean if u couldn’t stay home and take care of her why have a baby then. I mean it would definitely be applied to me also cause I’m a sahm as well and during the morning is when I do grocery shopping and stuff to get done and if I do this I won’t have time at all. And if I say no it’s like I feel it will be very awkward btw us especially when I take her (only) grandson since I’m the oldest daughter. How do I tell her I can’t do this for her? Without making her mad as if it’s my obligation especially when I’m her eyes I don’t work in the morning test I clean in the morning and take care of my son 24/7.