The way I see it she told her to go cry in the bathroom because no one is there to comfort her and as a way to distance both her and Akio from Ushio. Now why do this? Because they're waiting for Tomoya to get his act together and take responsibility for Ushio. They simply did it to leave the spot of parent open for him and to make Ushio miss her dad so to not take his place. Same reason why they didn't tell her about Nagisa. Think it probably hurt them a lot to not comfort her when she's sad or tell her about Nagisa etc so I'd rather give them credit for having that much faith in Tomoya for those 5 years.

I highly disagree with the whole he chose her just because she's so weak that she can die on him. Hell I'd describe Nagisa as weak because her body was weak, it's the truth so why shouldn't he say it? But he's making a point of how kind she was and strong willed even though her body was weak.