Cat help needed PLEASE!?

I have an 8 month old siamese cross cat who keeps pooing and weeing all around the house. She does this maybe twice a day, any other time she uses her litter tray. She is not desexed. I need help fixing this as if she keeps doing this my fiancee and his family wil make me get rid of her. She isnt sick, I think shes just being naughty or has some behavioural problem or something. I have tried changing the type of litter in the trays, she has her own tray, its in a corner away from where she eats, its cleaned regularly- I dont know what else to do! Please help!