After giving birth... (Rant)

Now, admittedly, this is petty on my part so I'm just stating that before anyone feels the need to tell me so.

I have two kids, third on the way. He is due in June. When i had my first I was in high school and my then boyfriend/now husband stayed with me for a week. After that his mother made him to back to his home. With my second, my MIL took two weeks off work... still dunno why. Now, with my third she says that my SIL will be in summer break (she is 15) and can come help with his birth. This has really irritated me. First, my family works from home so they would be my go to people. Second, why do I suddenly need help with #3. Third, having my SIL here is nice but it's another mouth to feed and mess to clean, not to mention she hypes up my two children. I realize the intentions were good, but I don't feel the need to babysit her teenager post partum. I may be too irritable but she did not care when I had my first at sixteen and was begging to have my husband stay to help, she did not care when I had my second and we had just moved out on our own. Now, I have a hard time believing that she cares because she tends to pawn her daughter off on us during the weekends so I feel like she just wants summer childcare.