1. 你们俩别跟我说 means "please don't tell me that..."
2. 摆那儿呢 means"(your son) is there", to emphasize the truth that they have already had a son.
3. 咬死了here means insist. When some one doesn't want to say sth he'll close his mouth tightly even bites his lips. That's why they use 咬死了 here, means bite your lips to death and do not say even one word to admit what you have done.;)