Depends on family.

My ex-mother-in-law and I get along great. She gets along with my wife and has asked our son if he would like to call her grandma. I'm very fortunate in that regard and recognize that most don't have nearly as positive an experience.

Your family includes whomever you choose to have it include.
There is no right or wrong.
Some people are great and should be part of your life....others are toxic and are best only seen from a distance or not at all.

My mother in law pushed the boundaries of our relationship before & after the death of her son, my husband, therefore she is now my ex mother in law, and that feels wonderful.

Ive never really thought of my ex's family as ex family. Except for my first ex. His dad was the only family I really knew and he was a twatwaffle way before his son was my ex. My most recent ex's family acts more like my family than his. I don't think they have even heard from him in over a year. I see his mom and dad every week and the sisters and brother at least once every month or so. We have had our disagreements but we are still family. As far as widowish stuff I have no experience so I can't really say one way or another with confidence.

I've only been divorced, and yes, after a divorce the ex's family typically stays loyal to him and cuts off the ex wife.
Often, that's a positive, though, if the in-laws were unpleasant or critical during the marriage.

But a widow's in-laws usually still see her as part of the family, and someone they should protect and help.