L.B.
Of course the kids "aren't crazy over" you - you are the disciplinarian. Which is sad that the entire role of disciplinarian fell to you. I am sorry but your husband needs to step up also and take over at least 50% of that from you.
Grandma needs to tell the child herself that she cannot live with her. Short of that, your husband needs to relay the message to the kids.
It is time for a family meeting with you, hubby, grandmother, bio-dad, and uncle. All of the adults need to be on the same page. If the kids are to continue living with you then every other adult in the family needs to give you 110% support and stop the under-mining.
Also, if your niece/nephew are not in counseling it is time. Family counseling for you all would help you all relate to other also.
It is a good thing that you do in caring for them. God Bless.