I think its more a problem that you know to the day how long you have been dating. Its a bit obsessional. And why are you worried about marriage even before you go into a relationship, most people don't go looking for marriage just someone they like and get on with. Your obsessing so yes its problem. No idea where your sister comes into this. Id strongly suggest that you don't date for a while and just see what happens. And get some counselling to deal with the feeling of grief you have regarding your mother.