How to deal with Mom's bf's introverted kid

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so my mom has had a boyfriend for about a year now, who has been family friend i was a child (he's fine, no issues there). Problem is, his kid, who I've also known for my entire life, is super socially isolated and just plain weird. He was a normal kid growing up but I think his parent's breakup (and maybe his mother's experience with schizophrenia) has affeted him negatively. He prefers to stay inside, doesn't respond to questions/participate in conversation, listens exclusively to classical music, is years behind in school (now "homeschooled") and as far as I know only plays video games in spare time. He is my sister's age, a few years younger than me, and I've moved out so I never see him when he is with my family.

My question is this: How do I get a socially disengaged person like that to open up and feel comfortable/social with me and my sister so we can help him be social and not feel creeped out?

I've tried playing video games with him, making conversation etc. and no dice. I am also worried he may try to fuck with my sis as she is popular and outgoing in high school and I don't want him creeping on her.

Help me help him?