My husband had an inappropriate friendship with another woman a couple years ago. The other woman was coming on strong to my husband and daughter with an obvious intent of replacing me in my family. After a while, I confronted her and asked her to back off - stop coming over all the time, stop buying my daughter stuff, stop sending your clothes to my house with my daughter, stop making my family treats, stop texting my husband - just STOP! She just got sneakier and kept it up behind my back. When I found out we had a HUGE family blow up and my daughter sided with the other woman. My daughter is about to marry the other woman's son - which was all part of the floozy's plan from the beginning. My daughter has told me that I will have to accept the floozy and her deadbeat son if I want to have a relationship with my daughter. The other woman had plans from way back when to have grandbabies with MY husband and now that is a good possibility since our children are about to get married. I don't want anything to do with the floozy or her son and I sure don't want to share grandkids with her but here we are. What do I do? Do I accept the floozy as part of the family or write off my daughter?
Again, I think you need to talk to a professional. You might be able to find family therapists who offer a free first session and possibly over a Skype or Zoom call. I think you should shop around and take as many free sessions as you can until you find a therapist that works for you and/or your family. That said, I think it's important to get your family on board as well but...
At the end of the day you can only control yourself. You may have some legal avenues to restrict this woman and her son from your home and property but those rights may be superseded by your husband and daughter if they decide to grant access. You probably have very few legal options to restrict your family aside from walking away yourself which I'm sure you don't want to do. You'll have to seek pr0fessional legal advise and/or look up what options you have granted to you by your state or country.
I think your time will be best served searching online for professional advice rather than seeking it here (as you can tell from the response from the other poster). And I think you should start sooner than later since I'm sure this situation will only get worse with time.
To answer your initial question though, ultimately, the choice is yours and yours alone. That said, I tend to lean toward maintaining positive relationships with my family so, personally, I would accept the woman as family. My view also stems from, whether you like it or not, your husband may want to maintain a relationship with your daughter and also view the woman as family if your daughter marries her son, so it may cause an additional rift with him if you choose not to accept the relationship.
Good luck to you and your family.