I deleated my start up disk

I got this computer from my sister - it had multiple users with passwords soninwas signing in as guest. I just wanted to reset the computer so i could start over and be admin.

I accidentally deleated my start up disk in recovery mode. Now i cannt get on - it starts in recovery mode. I tried to reinstall os x yositime but it says temporarily unavailble. Maybe bc my apple id is not recognized by the computer since it was previously my sisters. She doesnt remember what apple id she used so i dont know what to do! Help!