The picture of the two of them is whta makes Nika ask Bella about her sister the first time he's over her house to study. Because of her reaction, and the way Ayua reacts about Nika asking about Bella's sister, it's safe to assume something bad happened.
Because she appears to be in home care, and there's all those missed presents, it's be really weird if it was a stranger ...or former roommate.
And there's enough life support gear there, to consider that if she's in home care, they've set up for the long term. Bella talking to her knowing show won't get a response, the unopened presents etc, wishing she could see Nika's eye's someday, all suggest she's not waking up.