Mourning process for husband of mother?

Hi,

The husband of my mother (not my father, they’re divorced) passed away this morning. Although I do not consider this man my father (this was not the relationship we had) I respect him a lot and have known him for more than 14 years.

The shiva is going to be held in my mother’s apartment, where I still live; i was wondering what the customs are for this? Of course I will accompany my mother and his daughters and be there for them, but according to halacha am I expected to sit shiva and be in mourning as well?