I disagree. The sister was the entitled AH here. Many families have children with the same first name. I share a first name with my cousin, actually. OP using the name Morgan does not prevent her sister from using it in the future. And Morgan, in my opinion, is a much better name than Derek. Think of this from the OP's perspective: You are 7-9 months pregnant with your first child, told that you are going to need to be surgically sliced open to give birth and all your sister cares about is what name you use because she, for some idiotic reason, thinks that you using the name Morgan somehow means that she can't do the same thing.

No, "OMG they're going to cut you open? Is everything ok? Are you alright?" But, "Please don't name your kid Morgan."

Awful. Just terrible. The sister was being immature and given how difficult some pregnancies can be; imagine the OP having to endure morning sickness and back pains for months and then getting SLICED OPEN while being continually badgered by your sister over a baby name. I might have lost my temper with her too. I'm in immense physical pain and probably terrified at having to undergo a surgical procedure to give birth and you are here badgering me about a bloody name???