Maybe she is just away for an extended trip, or went to a school or something. MC could actually be a quick thinker and used sister to escape the shitstorm he found himself in last chapter. Technically she is a girl and you like/love family. So sister = girl I like/love. And no tragity tag as of yet so...
Well ok. Backstory which hopefully ends better than my past experience with such characters. I mean this manga is pretty lighthearted with a lower age rating so i guess it would have to end better than everything i last read.
Eh wth, i'm just realized in the last page that the chapter before this Al was staring at a photo lol
It can be guessed that the next chapter will be sad