You are an adult and a married woman. You should not need anyone’s “enthusiasm” beyond yours and your husband’s to bring a child into this world.
You stated that your mother was “sloshed” when she made the remark. Alcohol-addled individuals often make inappropriate comments. Who knows what she meant when she said it? It may be she was thinking about the pain your brother and sister-in-law are experiencing because of their infertility issue.
Because her comment was hurtful, I think you should let her know so she can clarify — if she even remembers saying it. And if she doesn’t, suggest she cut back on her drinking because memory lapses can be a symptom of a drinking problem.
You accepted the invitation, didn’t you? Because you did, you should have given the couple a gift. You did not have to take one to the party, but you should send something within a short time. (Suggestion: A nice picture frame to hold their engagement photo.)