Is it Grandmother’s Day?
You simply took the band-aid off, so she was going to find out anyway. When it comes to a simple miss-sent SMS, you’ll likely have both your sister (almost sure) and your mother (maybe) on your tail. You’d better start stocking up on baby shower presents to make up for it.
Get a dinosaur-themed onesie for your baby. That’s a certain winner. Another option is that you might claim that the message was intended for a completely unrelated person and not for your sister at all. There. Mom’s problem has been addressed till she gets in touch with her sister.