Non-Muslim Proposing to Muslim Woman
Question

                Assalam O Alaikum
I am a muslim. My sister has been proposed to by a man who is 
christian. She has agreed and they are now planning to get 
married. The man is willing to convert to Islam. I dont know 
what to do. Do I have any rights to resist this marriage? My 
parents are totally against this marriage and are deeply 
concerned, mainly due to the fact that the man is from a 
different culture and is not a relative.
Please advise me on what to do as I am very confused and 
time is running out very fast. I will await your reply anxiously. 
Thankyou.
Praise be to Allah.
It is not permissible for your sister to marry this man unless two conditions 
are fulfilled:
He must enter Islam.

Your father must agree because he is her guardian who is responsible 
for arranging her marriage.

If either of these conditions are not fulfilled, then try to stop this evil from 
happening, whatever it takes. 
And Allaah is the source of strength.