I don’t think enough people pay attention to Armin and Mikasa’s dynamic enough.
A lot of people assume Mikasa cares only about Eren but that’s not true. It’s just that Eren is, quite honestly, an idiot. Despite being “weak” Armin isn’t in harms way 80% of the time. But when he is Mikasa protects him.
In this scene when talking to Armin, in the manga, Mikasa asks first if Armin is hurt and even how his emotional state is before inquiring about Eren. Later in the same arc she refuses to leave him behind even though she had just started valuing her life she was willing to risk it for Armin.
Furthermore Mikasa trusts Armin with both her and Eren’s lives. And we all know how hard it is for Mikasa to trust other people to protect her brother. Even when Levi is beating Eren in front of her she does nothing because Armin told her not to. He grabs her but he’s certainly not strong enough to hold her back, she held herself back because she trust Armin’s judgement.
TL;DR:Eren is not the only family Mikasa has left. She has Armin too and she knows it.