I forgot everything about my appleid but i have imei and i can prove im owner appleid of that imei.how can i get my appleid

my sister bought iphone 5 1 year ago but she dont know much about iphone,ios,google account,... So she dont care about appleID, i didnt know that so when she has problem with appleID i dont know how to fix it because she dont know everything about her appleID. I just have imei now,and she told me that seller at cellphone store registed her appleID with her name is Nhung and i have something to prove my sister is owner of that account so i hope you can give me her appleid to unlock her phone