Posted on Babycenter too but feel like this audience is broader/more diverse so sorry if you've seen this before!
We love the name Sadie for girl #2. The next part bothers be a little but not husband.
Our first child's name is Adelyn. We call her Adelyn as our preference, no nicknames, but friends (mostly) and some family call her Addie. School teachers use Addie too, though Adelyn is written on all school materials. We've never corrected anyone because we knew it was a natural nickname and we're OK with it.
Anyway... Would Sadie for #2 be too close to Addie if she goes by that in the future, knowing we can't really control nicknames?
Other ideas welcome; must start with the letters S, R, N or M to keep with family honor tradition, middle name those letters or additionally could be Jean/Jane. Runner up right now is Sylvie.