This is a tough issue and I'm sorry your family is going through this on such a sensitive subject. There are a couple of considerations in your question. What is
it that your dad's family is asking your mom to "sign" over? In West Virginia, it may be a deed or a certificate of burial right. If that's the case, she may have a legal right to be buried there. You have a state cemetery association in West Virginia and they're very helpful. Call someone on their board of directors and they will be able to advise you accurately according to the state laws and your options. Their website page, which contains the names and phone numbers is [www.wvcfa.net/board_of_directors.htm] (http://www.wvcfa.net/board_of_directors.htm)
There are a few things you may want to think about:
1. If your mother does have a legal right and she is buried there against their wishes, will it create a different problem? The law says that they can't prohibit you from visiting your parents' graves, but they may try to make it uncomfortable for you to do so.
2. Is there some way you can find out why they are opposed to your mother's future burial there? It may be a concern that can be cleared up more easily than you realize. If you're not comfortable contacting them, maybe another family member who is, can do it on your behalf.
3. The other options are to have your mother buried in a different cemetery, which of course involves spending money to buy a grave.
4. If you don't want her buried in a different cemetery than your father, there is a more drastic option - your father's casket (and burial vault if there is one) could be disinterred and moved to another cemetery, where your mother could be buried next to him. There are significant costs involved with this, such as the cost of the disinterment, transportation of the casket (and vault) to the new cemetery, purchase of the new graves, and digging the new grave to "re-bury" the casket.
When you deal with cemeteries and family conflicts on where to bury someone, there are a lot of personal emotions involved. Peace of mind about a final "resting place" is important. I wish you and your family well and hope that a resolution comes easier than you expect.