What should i do if i really like my best friends younger sister?

You should let your friend know about it first, that way they will take into account that you have thought about them, rather then you just going ahead and asking out there sister you need to see if you have there approval over the relationship first.

basically, mention it, but very lightly. dont be like "DUDE I LOVE YOUR SISTER<3"

and if he seems to resent it, then keep away for a tiny bit.