I agree with everyone else who has said, if you want anything from this thread,, you need to be clearer, and try and answer some of the questions.
Can you explain the family tree a bit?
'Step -Grandmother' is not very clear what the relationship is.
How can you say there is "no back story" when you have a 16 year old who doesn't live with her mother, and a whole family refusing to share details of a funeral. There is clearly a much bigger issue that one argument with this 'step-grandmother's new partner.
Ultimately though, if someone isn't wanted at a funeral, by the dearest and dearest, the last thing you should be doing is helping her find a way to get there. Whatever the circumstances of them not wanting a person there, that is where we are, and encouraging her to turn up is very irresponsible.
Why not suggest taking the time to go somewhere and reflect, or maybe speak to a presbyter a her local Church (or whatever faith she might have) , or go to a peaceful place (woodland, beach, forest) , or somewhere she has a special memory with this lady. Go with her yourself and raise a glass / have an ice-cream / share some memories, / take some photos to look at and talk about.