Advice on this girl plzz?

Met her at my sisters party she's 22 I'm 20. We flirted all night and she seemed really into me and kissed me a couple times. She even gave me her number without me asking and told me to text her. the next day I texted her and asked her to dinner sometime and she said yes with a :). but then my sis told me she had a bf. My sis talked to her wither friend and she said she isn't breaking up with him. What gives? Do i just leave her alone until she texts me? Do I forget about her I really like her. she didnt mention having a boyfriend to me that whole night. i feel kinda down now cuz we hit it off so well