Maybe physically you didn't look like her brother, but when you talked to her or when you tried to protect her she could see Chris.

She quickly found incredible comfort in you, even after the events in the city, so as soon as you were able to escape the city and stay safe, she took refuge with you.

And you worried about her, especially when she kept getting into dangerous places, like, for example, chasing Umbrella.

"Why not?" She questioned when you started to reject her plans. "I'm of legal age, you can't deny me things."