Why don't the family have a consistent view of where Marta is from?

This is likely done to show that the family doesn't care to remember where she is from; they only care that she is from somewhere different from them.

Their ignorance about where she came from is one of many examples of the family seeing Marta as "the other"; they believe that she is not one of them and doesn't deserve Harlan's inheritance even though the movie indicates that she was Harlan's close friend and a good person.

1. They ask whether they can recover Harlan's estate if they argue in court that Marta had "undue influence" on him (To which the lawyer says that her only influence was working hard and endearing herself to him)
2. Ransom tells her at the end that she has taken his ancestral home (And Blanc points out that Harlan bought the home in the 1980s)
3. Jacob, who is consistently described as a Nazi, calls Marta an "anchor baby"
4. Before the family hears about the will, they tell Marta that they see her as part of their family and will make sure she gets money from Harlan's estate, then they turn against her when they Harlan gave it all to her
5. They threaten legal trouble for her mother, who is undocumented, if she doesn't renounce the money, indicating that they want to protect their family which does not include Marta and her family members
6. The night Harlan dies, we see that they have pulled her into an argument about legal immigration and earning success through hard work. It is demonstrated throughout the movie that none of the family were really "self-made" as they wanted to claim, but they consistently see themselves as entitled to what Harlan has earned
7. Marta was not invited to the funeral, but multiple family members tell her that they wanted her to come but were out-voted, implying that this may just be a lie that say to make themselves look good while they don't really think she had a place at the funeral with the family. Even if true, it is clear that a majority of the family didn't think she should be there
8. Meg calls Marta and tells her that Marta should renounce the money since it is the Thrombey family's money, and therefore didn't belong to Marta (And says immediately after that they consider Marta part of the family)

I'm sure there are many other examples I can't think of off the top of my head. They get it wrong to show the audience that they only need to know that she is not from the same place they are.