How can a lady be married to her husband for 20 years and not consider the husband's family as her own family?

Not everyone warms to the in-laws and different couples find their own comfort levels with extended family. 

I'm commenting because I'm not able to respond to individual posts. not everyone loves their spouse and that's sad because if you don't love them why would you marry them? Also this person is my sister-in-law and her husband myself my fiance and her husband's mother have been nothing but loving and inviting towards her. The mother has invited her into her house watched her kids taken out the dog when she was on vacation and gone over to visit make special gluten-free foods for her if she can't eat what everyone else is eating has been nothing but warm and loving towards her and this is how everyone gets treated.

That's not her choice alone. Much depends on how she is treated and, if not well, she knows she's an outsider and always will be.

Some people consider family to only be blood relatives. Others know better. 

Not every one loves their spouses family.  Mind your own business. I know in laws that fight like dogs and walk out of the home and never see each other again.

I've been married 15 years, and my husband's blood family is not my blood family.  Why do I say that?  Because we don't share a bloodline.

She is simply one of those who believes 

that "family" is strictly blood relations, not 

something you marry into. 

She only views relatives as blood related.