Yeah, definitely try to do this in person (again?), and probably best without the husband? Bring her around first. I don't know your sister, OP, or relationship between the two, so unless she normally flies off the handle all the time, and is just always stubborn (could be), her initial response to your request could just be shock/defense when she sees her normal order threatened. People seem to be that way--I know I am, heh. Maybe after a day or two and some time to consider, and bring it back to her again, she'll come around to working with you?
I think you're totally in the right and this needs to be the way you want to do it and if she isn't willing to meet you there, after your helping them out over the last several months.....well, tough titties for her.