Hey Kayclan,
I have a BPD mom and have worked with two people I know to be high functioning autistic people. Both these aliments involves a lack of empathy, but some high functioning autistic people can come around, i.e. they have low rather than no empathy.
We can't really diagnose on here, you'd need a specialist to do that. Also you've not given out enough info. Being affixed on one thing could also be OCD. But the poor interpersonal skills do map to autism (or Asperger). A key definer is that people with autism have a very logical approach, so they don't get Political satire. But by the same token some with Asperger are not anti social. See here:
https://www.theguardian.com/commentisfree/2018/jun/04/robert-white-comedian-asperger-britains-got-talentI hope that helps but the main value of a diagnosis is to bring about change, is your sister capable of change ?