I surprisingly liked it. I thought it'd be really weird and kind of make me uncomfortable. But it's surprisingly kind of cute and charming in its own creepy way. If this was with a guy then I could defiantly say this would probably be raising lots of alarms.
The big sister is probably my favorite character of the group. I do want her to be more social and hell, I wouldn't mind if she ended up even getting a girlfriend ( I say, girlfriend, cause It's Shoujo Ai and I prefer Yuri anyway)
I guess it wouldn't be that weird if in some odd years the loli and she started going out. Don't know when and how old the Loli would be ( Probably around 16 or so)
I do wish we the big sister was... I guess more fit is the way to say it. I mean I actually like her character design as is. It's different than the overly beautiful Big Sisters that always exist in anime. I do think she'd look quite nice though and her cosplaying with the girls would kind of make it more adorable.
Overall this is a surprisingly fun and not so creepy story. Now excuse me, I seem to have Chris Hansen from Dateline NBC at my door.