Well, if this is my SO and I see marriage in the future.... extremely.
When you marry someone, you are marrying their family. If they hate you before you get married, they will hate you after. If they dont get along with your SO, then you wont have a relationship with them. If thats what you want for your kids... then welp.
My mom went through this - my dads family treated her like ish and made her life hell. My bff's mother married her husband, and his family treated her like ish. Now we are sorta realizing that HE is a major source of the problems, but still.
Its just bad news.
Plus, I think that someone's family says a lot about them. After all, they are the one's who raised them.