I think it's crappy for anyone to say you're unsisterly for not doing whatever it is they are wanting you to do. I never ran into this type of situation exactly, but I know in my chapter there were several girls who were not in the best of financial situations and to say they were unsisterly for not contributing extra is just wrong.
We had a sister one time come in to an exec meeting saying we were all horrible sisters because we hadn't finished our scrapbook pages by her deadline. That made me mad........I would be especially mad if someone said the same because I couldn't afford to give any extra money to a special cause.