Do you think I’m being taken advantage of?

So I watch my husbands nephew every week day before I go into work. He’s 2 and I’ve been watching him for a bout a year. Well I work 30-35 hours a week, do online classes, and I’m a housewife with a busy husband so I have to do all the cooking and cleaning as well. Well I’m currently 5 months pregnant with my first baby and it’s getting harder to keep up. My husband keeps telling me how rude it would be to stop watching his nephew because then his sister would have to pay someone (keep in mind I do this for free) or she’d have to miss work. My husband past a comment about how his nephew is gonna be great help once Im big and pregnant and how he’s gonna help with the new baby..... like stop volunteering me to take care of this baby in addition to my new born child. He said his nephew is chill and doesn’t cause any problems. This is true but I’ve sacrificed a year of getting up way earlier than I have to watching the baby all morning and going into work long hours late at night while getting homework done and keeping up with house. I’m not even super close with my sister in law. I pretty much see her when she drops off and picks up the baby. My husband is pretty much telling me I don’t have a valid enough excuse to stop watching him yet because my sickness has stopped and I’m “not doing anything else but sleeping in the morning” I’m pregnant and tired dammit! I get no benefit of watching the kid other than being able to help out family and now I feel like they’re taking advantage of me. I’m so busy and exhausted all the time and I’m scared this baby is gonna come and I’m going to jump in on empty. 😔 how should I go about this situation?