That could possibly be so. Chisaki's feelings toward Tsumugu and his gramps are already that of family and beyond though. Recall her breakdown when the old fellow went to hospital. Tsumugu's been showing us affection towards Chisaki, but I don't think he does it much (if at all) while she's around. She was family around him for 5 years and he was more or less comfortable with their relationship thus far. His "is becoming pretty not enough" comment seems to be the first time he's said anything directly to Chisaki like that.

She may well feel guilt since she feels indebted to them, but at the same time I think she understands that such debts should not override your romantic feelings. She certainly feels uncomfortable now that Tsumugu knows she's met with Hikari. She doesn't like change after all, and this meeting kind of sets that off.