I think TootsNYC may be onto something. Perhaps one last attempt would be in order, just to make sure that the family understand the possible dangers (the fact that the poor girl won't be able to make herself understood if she suddenly falls ill or if there's an accident).
But if the family still don't want to take any action, I'm afraid you do have to drop it.
Well, it's not that I think she should focus on the dangers.
That's alarming and it shuts people down.
I think the OP should start focusing on the things they *can do*.
So instead of saying "I can't understand her," say, "I heard of a speech therapist."
Say "speech therapy would do wonders for her!"
See the difference?
And I think that I personally (if I felt as strongly as the OP has indicated she does, and if I felt as sure of the problem as she does) would make about 10 phone calls to find out of there's any low-cost way to have the child evaluated, and then I'd start mentioning that conversationally.
I might start saying stuff like "when you get her started on speech therapy" as if it's a given, but it just hasn't happened yet.
Start do my small part to create an atmosphere in which the idea of taking the kid to speech therapy is simply an obvious thing that will soon happen.