I think i just screwed my sister?

out of getting into a good college, my parents wanted her to go locally and she refused and said that when the acceptance letter comes she is leaving this town. So when the later came i hid it from her and now she had no choice but to go locally because she thought she was denied. The Sign up date for classes ended today so now she will not be able to go for at least the remainder of the summer! Should i tell her the truth or just put the letter in the mail again. Do you think the school will let her in if they think it is their fault for not getting her the letter?! any suggestions?