Feeling pressured

My boss has been pressuring me to do someone else job & it’s stressing me tf out. The person is capable of handling the job on their own as they have for the last 3 years. However, occasionally that person does need help & when that happen she asks for my help. BUT my boss just wants me to overstep a boundary that I’m not comfortable overstepping by just hoping on to her domain and doing some of the workload for her when it’s busy. I already cover for her in the morning because she comes in late (which idc or mind to do) however, I don’t want her to feel like I’m overstepping because I personally get upset when she hops on and does my work for me (especially when I pretty much have nothing to work on).

Idk how to handle this situation at all in fear that my boss will be upset with me or even her getting upset with me. I really can’t have that stress on me rn as I’m 31 weeks pregnant.