Is a wife considered sinful if she asked for divorce because her husband married another woman?
A second marriage is lawful, and so it is impermissible for you to ask for divorce because of that. If you are certain that you won`t be able to deliver Allah`s rights and those of your husband`s , then it is permissible for you to ask for Khul`a (Divorce on the demand of the wife). However, you should exercise patience since after difficulties, there is easiness by the will of Allah.
Is it permissible for a woman to say that she loves her husband more than her father?
It is impermissible for her to say so since this would hurt her father deep down. Allah, The Almighty, Says in this regard (What means): "And that ye be kind to parents." [Al-Isra/23]. Saying such a thing to her father isn`t an act of kindness since she should keep it to herself even if she felt that way.
Does fasting on behalf of a deceased person permissible?
Fasting on belhaf of a deceased person is permissible, since the Prophet (PBUH) said: "Whoever dies while he still has some fasts to make up (of the days of Ramadan), then his heir (any of them) should fast on his behalf." [Agreed upon]. The previous answer is for making up missed obligatory fasts on behalf of the deceased. But if the fasting on behlaf of the deceased was for performing a voluntary acts of devotion such as fasting....is permissible as adopted by the majority of Muslim scholars and based on the above hadith as they stated "Every good dead intended to be on behalf of the deceased its reward will reach the latter." And Allah Knows Best.
Is it permissible for a woman who has become pure before the end of forty days of confinement to perform different acts of worship? and is it lawful for her husband to have sex with her?
Once postpartum bleeding ceases and she is certain that it won`t happen again, she becomes pure, and so she should perform Ghusl (ritual bath), pray, and fast. She can also have sex with her husband even before the end of (40) days because the minimum of postpartum bleeding is a moment, and the maximum is (60) days. However, the postpartum bleeding for the majority of women is (40) days, but this does not apply to rest of them.