I'm not sure about the lolicon tag. Natsumi does not seem to see the little sister that way. Love triangle tag does not seem needed either. Possibly the incest tag does need to be added though. Probably. Thank you for the chapter.
If it needs the incest tag, how can it not need the love triangle tag as well? Either that or poly as it's kinda hinted at. If the little sister's feelings toward her sister are romantic, hence the need for the incest tag, then it must constitute a love triangle, no?
I agree you might need poly in the future but a love triangle consists of "a" loving "b" who loves "c" that might love "a". ...... Neither of the other two even see the little sister as a romantic partner.