Your mother’s will is not valid if she is mentally incapable when she makes it. Often someone is mentally incapable because of dementia such as Alzheimer’s disease. However, the fact that someone has a mental disability or disorder does not in itself mean that they are mentally incapable of making a will.
To draw up a valid will, your mother must understand what a will is, the effect of making one, the extent and value of her property, and any expectations her family, friends or others might have that they will be left something. If she is only mentally incapable at certain times, she can make a valid will during a lucid period. It is wise to have a doctor or psychiatrist there who certifies that she was capable at the time.
Your mother should also consider making lasting powers of attorney (LPAs) while she has the mental capacity to do so. If she becomes unable to deal with her finances or make decisions about her health and welfare, this means someone she trusts — the ‘attorney’ she appoints in her LPAs — can deal with these areas for her.