Okay , let us redefine this question for ar desi audience
Would ya marry , your best friend or friend’s sister/brother ?
if he/she is good and belonging to good family then why not. it is kind of awkward for me to have a date with friend's sister. i dont think any bro will bear it. but one can ask politely and very nice manner that he likes her and want to propose her by parents. but asking for date is big NO NO in my dictionary.
I only have one brother and all his friends are old and ugly :(
One married sister who has very nice (not physical featured mainly) Hijaban friends but either all are married or she's too stubborn to introduce them to me.
My bestfriend is married to her brothers (cousin) bestfriend. The guys have been friends for like forever (though the guy moved to UK for some time and has been ladoo over her since he was 15 years old hehe) They have been happily married for 7 years and together (engaged) for 9 1/2 years. MA
It's all about your intentions and how you approach.
If you are going to go behind your friend's back and date his sister, then things will just not work out. Because at the end of the day, you are indirectly letting the family and the girl know that you are not honest and can't be trusted.
However, if you approach your friend in a respectable manner, then I am sure he will give it a thought.
Dating her would be a big no no, so if this is what you are after then forget it. If you want to send a proposal respectfully, then you have every right to approach her family.