What can I do if sister in law has claimed that my engagement ring was promised to her as a child by my husband's grandmother?

All of the allegations seem very hard to prove. Your sister in law would have a difficult time in court assuming her grandmother is dead. Assuming the grandfather was incompetent when he gave it to your husband, the gift might be overturned. It seems unlikely that either your husband or sister in law has a good claim to the ring over any other heirs.

Well the ring was your future father in laws property. He can do what he wants with it. It does not matter what the grandmother said at one time. promises that are not completed are not binding. The sister in law has no legal claim on the ring unless the grandmother gave it to her in her will. If the father was in his right mind at the time he gave the ring to your future husband then it is yours.

It depends on whether the grandmother is living, and if not, whether she had a will, and if so, who she left the ring to. Her promise to give it to her granddaughter does not give the granddaughter a legal right to the ring. However, if she is alive, it is her to give it to whomever she wishes. If she is dead, and the grandfather inherited it (which is likely), the grandfather can give it to whomever he pleases (provided he has the mental capacity to conduct business). If he gave it to your husband (you say use it, which is not the same as a gift), it becomes his property, then yours when he give it to you. Legally, it may very well be yours. However, if you want a happy marriage and good relations with your husband's family, you should be gracious and let the sister have it.

If he gave it to you it is marital proper and half yours.

I don't know what your husband was allowed to do. If he had a right to the ring he could give it to you. he. Did give it to you. Did he have the right? If he did, it is your ring, doesn't matter what he promised someone else some other time.

It's your ring. Your husband gave it to you and the fact that someone else said something to someone else many years ago is of no legal significance.

There are many aspects of this particular question which I believe should be taken into account that are not legal in nature. I would do so that they grandmother has died and that all of her jewelry and possessions have been given your grandfather. Once that occurs they are his property and not the grandmother's regardless of promises which may have been made to the grand daughter unless they were in writing. The real question is are you willing to live with the consequences of its return, either your loss of the ring or the continuation of your relationship with your sister-in-law into the future if she does not receive the ring. Legally will appear that the ring one from grandmother to grandfather who is gifted it to your husband and that the sister in law's claim legally is not particularly well-founded. That the preliminary opinion, however, is made based on the facts which have been disclosed or assumed. I would repeat the believe that the emotional issues here are more important than legal ones. No ring is worth the breaking up of the family.

If the ring was promised to your sister in law by her grandmother, which sounds very reasonable since a grandmother should want her grandchild to have her ring so as to keep it in the family, then the sister in law should get the ring. Just get your husband to get you your own ring. It sure it not a basis to start an inner family feud and create problems between your husband and his sister nor between you and your new sister in law.