is it okay to be in a relationship with my friends younger sister?

so im 22 and my friend is 26. we met about 3 years ago in a community college class. we started studying together and playin basketball whenever we had class. after that we somehow ended up having 2-3 more classes together both by coincidence then by us signing up for the same class. well his younger sister who is the same age as I(she is one day older than me) i've always liked but she had a boyfriend at the time. i would always party with her, her bro(my friend) and her bf. so i talked to her here and there, not much because i didnt want to intrude on her bf. just a couple of months ago she broke up with her bf. and she has been hitting on me A LOT!

for example last night she kept teasin me how she's "older than me" by one day and that she'd be the man of the relationship(just playing around). and she would sit on my lap multiple time and she took my hands and put them on her ass. and told me to squeeze to see how firm they were. i was kinda uncomfortable because her brother was at the party but he was downstairs. i mean he has seen us sit really close together and she doesnt seem to care what he thinks. she even gave me a choice to next to her or her brother(my firend) at the movies. shes like "well if you wanna sit by him i understand if you swing that way" and i was just like no no i wanna sit by you.

so whole point of the story is ive know my friend and his sis for about 3 years, i always come to there house prolly every once a month to watch sports and just wondering would it be weird if i started dating her. i really like her, it wouldnt be a fling or anything. she got over her bf and i actually just got over my ex couple months back as well. i know i'd treat her sooo much better than her ex because he just used her to party n drink n stuff and he never even got her a bday present. i know exactly what she'd want for her bday and i'd even make it hand made! well thanks for your opinions