Hello
It is a very interesting question that even after 10-12 years of sexual contract one didn't got
HIV infection or didn't show any sign or symptoms , may be due to these possibilities , these include:
1 Your mother's partners may have taken precaution every time during sex i.e. protected sex.
2 He may have taken anti retroviral treatment before any sexual contact .
3 He may have not got any HIV infection due to any idiopathic reason .
Your sister got HIV infection from your mother , it is established .
As you mentioned that he does not take care of himself , you may be wrong , sometimes I have seen my so many addicted patient that they take care of each and every thing , even more careful than normal person.
But no doubt this is an case of an academic interest .
Hope that you will take care of your mother and sister .
Good luck.