Surprisingly mc doesn't seem to want his sister for any weird, freaky reasons. Its seems his parents might be separated and he just wants a little sister to love and appreciate. might even b a bit wholesome. excited to see where this goes.
I think he either tried to make a childhood friend as little sister but the friend liked him so refused to be his little sister in order to be something else but by making this choice made mc deserd her or their parents may be separated and she choose to stay with the dad leaving him really disappointed to the point where mc dont aknowledge her as his sister anymore. Still too early to be sure.