Xplantdad it sounds like your sister may be asbergers syndrome person. These are very smart people who actually are mildly autistic and have absolutely no ability to deal with people. They are blind to any nonverbal signals from other people so would not know they were being offensive. I firmly believe that if some one is nasty and miserable I won't have anything to do with them. But generally I believe never blame malice as the motive for behavior. When stupidity vcovers it.
I am so happy that your in-law called and that the two of you were able to talk. Could you tell us more about what exactly this person apologized to you for? Could you tell us more about your conversation? If you want to keep that private, I'd certainly understand! Anyway, I am very glad that you are feeling a bit better!