Yeah, I would definitely check with your travel office. I checked some things out with our computer system, and it looks like United employees pay a $25 service charge to fly on Cape Air, and we can NOT bring a companion with us. I would imagine there to be some reciprocity. On the other hand, I did see that Cape Air employees fly at SA5X, which is the same priority as other United Express carriers (such as SkyWest). Also, when I was with SkyWest, they did allow for an enrolled friend if not married, so there's that too.

Hard to say for sure, but I would have to guess that your sister would not be able to travel with you on your benefits. Check with your pass travel office to be sure, since we don't have any active Cape Air members around here.