Unsupportive Sister

Samantha
So my sister has had two beautiful kids already that I am always looking after. I have just been married for a year and we found out we were pregnant I was not trying nor was I preventing as I have PCOS and I am overweight so 10 months into my marriage I wouldn't have ever guessed I would have fallen pregnant and I am truly blessed. 
My older sister on the other hand always made jokes that I could only fall pregnant when she's finished as she needs my help with her kids. I don't mind looking after her kids I look after them always and ensure they are cleaned fed and pretty much all my sister has to do is relax and take them home to bed. 
When I announced I was pregnant her first response was "so does that mean you can't look after my kids" and she continued to go on talking about what if she falls pregnant at the same time as she has been trying before I found out I was pregnant. I love my sister and I don't really need her to look after my kids as I think I have pretty much perfected it with hers lol but I wanted her to care or be excited especially cause she knows I could struggle getting pregnant and this is a true blessing. Am I overreacting because of my hormones or is my sister unhappy that I got pregnant before her?