Someone has stolen my cat...

Hello and I'm new to this sight and I think someone has stolen my cat because my cats are brother and sister so they always stay together and from thuserday 28th of may 2015 my cat went missing and sneeky shbeas brother has been crying for the past nights so me and my sister when round some houses and we did not find any but when me my nan and my sister when round to the same house on Saturday 30th may 2015 at 10 to 3 and the gates where there and the car so we spoke to the house owner and he had a answer for every question and he said that he had a cat the same as Sheba and he had not even seen the poster we had and when we said sorry to bother you have you seen my cat and he agent strate into his shed and got sneekys coller and gave it to us and he also said that he sees them all the time in his garden so that made my and nan and my sisster suspishios and on the same day as when we where sospishios my mum called the RSPCA
And I don't know what to do plz help me :-(

I'm not sure what we can do as it's difficult to understand the exact situation.

Perhaps you may like to read this article and the comments about it for help with finding a lost cat.