I'm an outgoing, secure, extroverted type (31F). Just met my boyfriend's (29) 21-year-old sister for the first time. She came to visit for 10 days. 

I knew going in she had diagnosed/medicated anxiety. She flew in and things seemed fine for a few days. The three of us were having dinner here and there and hanging out at his place here and there. I tried really hard to embrace her- asking what she liked and what I should cook for Christmas dinner, getting her Christmas presents, bringing her nice bedding for her brother's futon, asking about college, etc. 

Next thing I know, she's sad and there's some kind of argument about my presence so I offer to leave. It's later reported that she feels like a third wheel and "doesn't know how to act around me" (or any stranger).

I dont think this is personal and I do feel it's extreme social anxiety.  How can I bond with her and make her feel comfortable around me?