Babysitting at 9 months pregnant.?

I plan on working up until I go into labor. I’m a manager at a retail store. I’m able to sit at my job and it’s pretty low stress. I do watch my niece every day while her mom goes to work and I work afternoons and evenings. This means I don’t really get days to myself. Working in retail means I have heavy hours on the weekends and hours are sprinkled through the week. I don’t get paid to watch my niece. I just do it cause my sister in law has no one else to watch her 2 year old. I’m only 18 weeks right now and due in March. I’m planning on telling my sister in law I can’t do it anymore after my baby shower mid February. The problem is my husband thinks I’m being selfish because if I can work, I can help her with her kid. I’m already tired all the time. I don’t have to be up early for my job but I’m up at 7am all week to watch my niece. I just feel like I need a month or so to prepare for my baby and get rest before she comes. This is my first child so I don’t know how I’ll be feeling but my husband is under the impression that I can watch everyone’s kids for free while preparing for a newborn. He doesn’t realize how tired I am already and I’m not even half way there yet.... did you take time to prepare for your baby? Do you think it’s selfish that I want time to myself before my baby comes? The arguement is that my niece is easy to watch and though she is a good kid, my entire life is about to change and I need time to get myself ready. Even if that means sleeping and extra few hours.

And just to add, I am also In school and plan to continue before, during and after childbirth since it’s online.