I'm not really crazy about ladys but the day i saw my friend's senior sister, a feeling in i never felt before came uppon me. but to say d truth i am really in love with her, but someting happened one day, she came to my room to ask if i am interested in a relationship with her, my friend heard about this and he is not happy about it. which one should i choose, my best friend or his senior sister, my lover?

the question of who do you choose should'nt arise . Rather it should be " How you should handle it"
Its quite simple. Just prevail on ur friend 2 accept ur new status as a potential Inlaw.
He's only being protective of his sista.
U would have done the same, i will do same too,
Your friend for sure will threaten fire and brimstones, u must be ready 2 tolerate bulshit if you hope to win. Set your mind right and you'd disarm him.
Dont ever threaten 2 fight else, you'll be biting more than you can chew.
Gudluk

THERE IS NO RULE AGAINST THIS.BUT CAN YOU HANDLE IT? REMEMBER SHE IS YOUR FRIENDS SISTER.
YOU STAND TO LOSE A LOT IF YOUR INTENTIONS ARE NOT GENUINE AND HER BEING OLDER THAN YOU
EASILY TRANSLATES THAT THE CHANCES OF YOU TWO ENDING UP TOGETHER FOR GOOD ARE VERY SLIM.

IF YOUR ATTRACTION TO HER LACKS SUBSTANCE THEN YOU MUST TRY TO GET OVER IT. THE TWIST IS,
SHE IS INTERESTED IN YOU TOO. IT IS A TRICKY SITUATION THAT DOESN'T HOLD MUCH FOR YOU.

tulburg:

I'm not really crazy about ladys but the day i saw my friend's senior sister, a feeling in i never felt before came uppon me. but to say d truth i am really in love with her, but someting happened one day, she came to my room to ask if i am interested in a relationship with her, my friend heard about this and he is not happy about it. which one should i choose, my best friend or his senior sister, my lover?

From your post i quote "my lover" U have already started something with her so why are u still confused on who u shud choose,u have made your choice.Your lover.