I totally agree that they should not have asked or shared the name ahead of time.

But the gracious reply from someone who is single, not pregnant, and may never have a son, in reply to someone else in the family who is currently pregnant, is "I love that name! You have great taste. I reserve the right to still use it if and when my time comes to have a boy." Or something along those lines. The SIL's response sounds dramatic and entitled, which is bullying behavior.

Exceptions in my opinion are when the name is held in reserve as an honor name (say, SIL was already in a relationship and Grayson was an family name on her partner's side) or was previously the name of a child lost in miscarriage or stillbirth by another member of the family, and so on.