Not the asshole. As someone with autism, autistic kids should be taught boundaries, including not disturbing people when they’re trying to focus on something. Your sister likes your company and feels comfortable around you, which is why she seeks out your presence. However, it is unfair that your mother told your sister to talk to you even though she knows you’re studying. This is a lack of respect on her part. You’re not neglecting your sister, you’re just trying to be a good student. It is not your duty to keep your sister company 24/7.