HELP!!!!! My sister is 18, her boyfriend is 18. She tells me they are getting married and wants me to go.?

if you really think she is making an unwise decision, i would tell her what you think and that you cannot support them and that you feel uncomfortable being their witness. and that if they insist on doing this, they will have to find a different witness because as much as you love her, you do not agree with them getting married.