I'm not going to allow my in-laws to treat my cat like he is theirs
Why don't you want to share your cat with your in-laws?
I will not allow people to take my cat from me.
If they are trying to take away your cat then you have every right to be angry. You should firmly tell them that the cat is not theirs.
I already told them that I don't want them to take my cat.
Your cat is yours and you are correct to let them know. You could let them still pet it and socialize with it if you wanted. There's no harm in allowing them to still see it when they visit from time to time.