if ever there were mixed emotions!
so sorry for your loss and congrats on niece - not sure what to say
we have a version of this situation other way round, after our Isaac was born a friend down the road had a baby Isaac, who suddenly died, only 3 months old (when ours was 10 months old).
I felt so weird, coz we had the same name - and every time our boy has a birthday I always feel sad and think theirs won't....
really tough - best for now just to send a card and let them know how happy you are for them.
as they will be very busy anyway, you can say that you don't want to be crowding them, would she mind letting you know when she's up for visits.
I'd actually talk to her on the phone and tell her about your MC before you see the new baby, explain to her that you really want to see them and can't wait, but will probably be very emotional and you'd rather she knew this beforehand.
then when it's time to see them ask if it could be just you (so no other family members/friends) because you don't want others to know or feel embarrassed.
if you have a good relationship, she'll be lovely and understanding I'm sure!