I hear you...
I stayed on at the same school for grad school. I could have been a great resource for my chapter, and I would have been happy to help out, if they'd only treated me with the respect due an alum - instead, when I went back to help out with rush, the chapter president was extremely rude to me.
You may be an undergrad alum, but you're still an alum, and deserve respect. You're not "pretending to be active" - you're an alum in the area who chooses to be involved with your chapter - and your sisters have to recognize that.
Take care.
