www.doujinshi.org lists this as Morning x Morning 3, but it's wrong. Morning x Morning 3 is the one posted in /g/847485/caa99d20b0/. There's a 3 in the cover of this doujinshi but it's where it says "Sister No. 3". No idea if the 12 sisters are numbered in some place or if it's the 3rd favorite of the author or something like that.
Surely the sisters are numbered. 1st 可憐/Karen 2nd 花穂/Kaho 3rd 衛/Mamoru 4th 咲耶/Sakuya 5th 雛子/Hinako 6th 鞠絵/Marie 7th 白雪/Shirayuki 8th 鈴凛/Rinrin 9th 千影/Chikage 10th 春歌/Haruka 11th 四葉/Yotsuba 12th 亞里亞/Aria At first there were only 9 sisters. And then the three (10th to 12th) sisters were added.