Assuming that your mother has passed, besides whether it is wajib for you or not, it would be the most noble and most loyal thing for you to do. Your mother might have forgotten, or unintentionally neglected, or gone through some difficulties, and did not perform her acts of worship.
You, the loyal son, are able to compensate that and the reward and blessings will not only go to her, but to you as well. In some hadiths it says there are some children who are loyal to their parents while alive, and not loyal when they die. Being not loyal is not doing dua or prayers or good deeds for them.
If you cannot roughly estimate, or have no idea, you have two options:
1. Pray what you can, as long as you are healthy and have time. The bonus is it's extra 'ibadah for you, which is a wonderful thing.
2. You can hire someone to do the 'ibadaat on behalf of your mother. There are many poor mu'mineen/mu'minaat who live off doing 'ibaadaat hire. Pay for whatever you are financially capable of.
Yes it is permissible for husband's father to marry the mother-in-law of his son if she is widowed or divorced. Wife is Mahram for her father-in-law only not for his sons and husband is Mahram for his mother-in-law only not for her other daughters. This rule of being Mahram does not include the parents of the husband and wife.
'Your mother-in-law must observe Hijab from your father but not from you. Obviously, when they are non-Mahram to each other, marriage between them is permissible.
If the father or grandfather are still alive, they are still her shar'i guardian, whether they play a role in her life or not. Unless of course she reaches out to them, and for an unjustifiable reason refuse to give their consent, in such a circumstance the case would be different.
However, assuming that they are still alive, she must obtain their consent.
If they have passed, for marriage to be legitimate, she does not need to seek consent, however from a social/moral perspective, she should aim at having family involved, and not pursue an ongoing relationship through a permanent marriage, and not temporary.
And Allah knows best.
In the case that her father grandfather and If we were to answer jurisprudentially, if her father and grandfather
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