I keep wondering the fact how Tsubasa's mother's pupils are just plain black. There's no glare in them at all, and she's the very first character of such eyes. Her demeanor is also very possessive, and you could see that Tsubasa's grandmother even was looking her daughter with worrisome eyes while she was talking about his son in the late chapter.
All things considered, I think those pupils are the author's way of telling us that she suffers from a serious mental disorder, and since she has a lot of authority and power (and just plainly japanese social culture(hierarchy), it's hard for other people to step up, and tell her that she needs some mental help also. Also it doesn't help since it seems she's the head of their family, since her family/she has a lot more wealth than her husband does (or that's my impression at least), so it's even that harder for other people to say how they really think.