What happens if I break my At&t contract?

I want to jailbreak my iPhone 4s but I know it will break the contract with them. Will they know that i jailbreak once I jailbreak it? What will happen if The contract is over? Will it just be my contract or will it affect my other family members that has At@t? Even if I unjailbreak it will they know?

They won't know that you've jailbroken the phone
But if you are really worried about it then you can pay a settlement fee to get out of your contract and it won't effect your family members