Please help me everyone.......what should i do???i am very depressed....

actually,yesterday night, at the middle of the night,i suddenly woke up and heard voices in my sis room.....i went there and saw mom,dad and sis there.....actually, there is a kind of ventilator above my hall which has the terrace above and which is very close to my sis room.....my mom had gone mental suddenly and was saying,almost in crying that why is the ventilator over there.???she had seen something but was not explaining it...we all were very scared as what happened to her??? also,we have 2 floors in our house..then, after 5 mins, we calmed her madness and then she told that there was a guy from the 2nd floor was watching into my sis room from the terrace as it is easily visible...and when she saw this, she went into my sis room and found her all naked and lights on.....andshe was telling that my sis was in love with the boyand was purposely naked....actually, i also suddenly saw a shadow upstairs when i came...though my dad went very upset in the other room after calming mom, but me, my mom and sis were all there only....and mom was terribly angry and upset and today also,she is very rude to us and upset...and she onlyfinds my sis wrong..while sometimes, my sis has the habit to turn on the lights and sleep and naked thing, you know.....who is right???? please forgive me for writing this question and it is too long...but what should i do to restore my mom faith in her as i know my sis can never do a thing like this,seriously.........PLZ HELP ME.......PLZ....