I love Mirabelle and I really love the nickname Mira!! I think It's a wonderful tribute as well...I wouldn't not name her this because it dosen't go with Hollyn, you may be looking a little too far into it,and have major regrets down the road. Good Luck with Everything!!!
I kind of like the name Mirabel but I think it sounds more Spanish than Italian, so if you are looking to honor your Italian heritage I'd probably go with Mirabella, even if it is a little more frou-frou. (Then again, I like frou-frou, lol) By the same token, I really don't think either of those options "fits" with the name Hollyn. How about Mirelle? You could still call her Mira too. :)
Love it.
Mirabel Regina is amazing!
Hollyn, Nicolas, & Mira(bel) are perfect for siblings too.