Ironically blood is thicker than water actually means the opposite of what everyone thinks which makes it appropriate for this post.

“The blood of the covenant is thicker than the water of the womb.” This actually means that blood shed in battle bonds soldiers more strongly than simple genetics. Although we commonly use it to suggest the strength of family ties, it doesn't refer to family at all.

The term family seems to give people the idea that they can act any way they want towards you and you have to accept it. In this case it is your husbands family so really any action needs to come from him or else you come off as the crazy wife without his backing. Tell him how you feel and ask him how he thinks it should be handled. Together you can come up with something that doesn't put him in the middle and that makes you feel more comfortable.