1) Your experience in a service organization shows your dedication to philanthropy and your experience as a leader!
2)
From my experience, no sorority makes mixers mandatory. As CutiePie mentioned about DG, many sororities have a system to clarify what is and is not madatory. For example, Alpha Gam has a Red, Buff, and Green coded system.
3) I am sorry to hear about your lil sis. That is awful and uncalled for! ...Want me to kick their a**?