I guess I'm not a very kind person. I'd send the quilt top along with the name of a few LA's in her area and a note:
Dear [sister],
I hope this quilt brings you as much joy in the years to come as it did for me to make it. I really wasn't sure how you might like it quilted, so I'm sending it along with a few names of places I googled (or perhaps you know of another) so you can get it quilted exactly to your liking. If you think of it, I'd love to see a picture of our collaboration when it's all finished.
Love,
me
___
It would be one thing if you routinely quilted them yourself, but the fact that someone is going to pay for the LA service ... may as well be her. Plus, how awful would it be if you sent the thing to her & she hates the quilting! Or turns around & sells it for cash. Your skill & gift was to piece the top for her. If you want to send the supplies for batting, backing & binding, that seems reasonable to me (though not necessary), but as for me personally, I wouldn't pay to send it out to a LA.
EDIT: Or I like Tothill's suggestion -- though I'd tell her to contact he LA directly & when they had decided on a design & coordinated payment, I'd take care of picking it up & shipping it to her.