Goodness, why assume it’s a joke? How painful this must be if it’s not. Is it worth that assumption?
I remember years ago visiting a former classmate of mine from seminary who lived in Israel. She became engaged to a working boy, and her younger sister, who was in elementary school (!!!) talked with me privately about her worries about how her sister’s choice would affect her in shidduchim (years later). It’s no joke. These are real concerns, not to be taken lightly.
What your sister is experiencing does put you in an awkward position. Such behavior on your parents part, can and will affect the choice you make with regard to a spouse. You’re fearful of standing up to them now. Why assume it will be any different later? What if you met an amazing learning boy, and only later found out he planned to work after a year or two? You might pass him up because of a traumatic memory of maltreatment of another. As tough as this might be, I would not suggest turning a blind eye. If you must, have your sister initiate the call to the rav to request his intervention.