Being sad for your babies who are not with you doesn't make you a bad person. There is no
right way to feel. You lost something tremendous when your babies died, and sometimes we don't really realize just how big it is until someone else's joy shows us. Being sad for you doesn't mean you aren't happy for them. And it is okay for you to feel this way.
I joined JM the day a good friend of mine gave birth to her little girl. We had been pregnant together and I was so overwhelmed with grief for my Cora. I felt the same way you do, I felt like I was a bad person for feeling that way. I was overjoyed that Abby arrived safely, but I wished it was something we were experiencing together.
It is courageous of you to buy your sisters-in-law presents. Soon you will come to love the babies for themselves instead of just seeing not-your-baby.