ANSWER:

You have the right to defend yourself in front of him however, your cousin has no right to mix with your husband in the first place as she is non-mahram to him! It seems that the whole family is not abiding by sharia and you are freely mixing with cousins and in laws and this is why there are many problems because of that and your distance away from Allah’s laws. 

Sheikh,my cousin tries to take the spotlight in many situation. it’s like we are in a group chatting she will attack me and insult me by pointing out my flaws in front of everyone. Also in front of my husband. She tries to put me down.but she do that all in a very playful way. But my husband doesn’t get the hint that she is kinda mischievous. So i should aware him Or is it as equal to backbiting?