She's probably almost certainly of the "If I confess and get rejected, I'll have nowhere to go but I wouldn't be able to stay!" mindset.

So the two of them are suppressing their feelings on both ends. Tsumugu doesn't want to make her uncomfortable by pressing things further, and Chisaki doesn't want to lose the only family she has left. He started loving her when she stopped crying all the time (and over time things gradually strengthened) and she started admiring him when they began high school.

Hikari's reappearance won't make of mess of things, I think it will finally push them in the right direction. Chisaki, who hasn't changed emotionally these five years, will slowly realize that she can't keep staying this way, caught in standby mode. Tsumugu will see Hikari as a threat to Chisaki's attention, and might actually make a move. He's already started by consoling her and telling her she's pretty.