1. There will be no problem for her act of attempting t commit suicide, if she is saved unhurt.
2. there will be serious problem for your relative if she finally succeeds in committing suicide. He will be surely taken under custody and charge of abatement to commit suicide may be imposed on him if inlaws so complaints to police.
3. It will be prudent on his part to audio/video record all such threats given by her continuously preferably with the admission that she has already attempted such acts and your relative has saved her.
4. Thereafter your relative can lodge a police complaint alleging her threats of committing suicide on trivial issues.
5. Such attempting to take extreme measure can be the result of psychiatric problem for which she should first be taken to a psychiatrist and if his prescription mentions such symptom of psychiatric ailment then ty will save you to a great extent later on since you shall be able to prove that she had this psychological ailment of committing suicide.