Can there ever be too many cons? Lately I've seen a son-con, the usual bro-con and sis-con, mother-con and now a daughter and father-con duo. The cons are coming to get us all!!
I have the feeling that the mature and big-looking sister is wet all over and makes those two jealous or maybe the two of them prepared some cosplay for her
I'd guess the locked space has to do with his parents. I don't remember if they've been mentioned at all. The only other thing that comes to mind would be lost memories from previous lives
How fast did she just become best girl? Maybe part also lays in Rui showing interest in another guy and absolutely none in the brother. Her looking so much down on him can never be a good thing in any kind of relationship but I still wonder why she felt so pissed of him gawking up her sister...