cannot remember my activation lock

I have an used ipad that i bought and it worked perfectly until my cousin for some how he broke the ipad and i brought it to store to fix it. They reset and all but when i do setting they ask for Activation Lock and no matter how hard i tried with my current apple id or my sister apple id, none of those works out. I don't know what to do. i don't know how my cousin got another apple id. now im stuck with useless Ipad. Anyone knows how to resolve this issue ?