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 the guardian to give Sadaqa (voluntary charity) from the money of the orphans?

It is impermissible for the guardian to donate from the money of the orphans because he is entrusted with the safekeeping of that money, and is prohibited from donating it.

I have been married for five years. My husband has two sons from his first marriage. Nevertheless, I took care of his children, and gave birth to a baby boy who is now three years old. Unfortunately, we keep fighting all the time; sometimes over his two sons and sometimes over his family. If I serve his family members and praise them, he treats me kindly. If I don`t, he turns my life into living hell. What should I do?

All perfect praise be to Allah, The Lord of The Worlds, and may His Peace and Blessings be upon our Prophet Muhammad and upon all of his family and companions.

Spouses should live with each other on a footing of kindness and equity. In your case, constructive dialogue is the best course of action. We advise you to be patient, commit your affair to Allah and ask Him to help you. We also recommend that you seek the help of the people of goodness and make them fix things between you and your husband. If all attempts for reconciliation fail then go to court, but we remind you that a problem, which could be solved with a beautiful word, is no problem at all, so praise family and children and try your best to win their hearts. And Allah The Almighty Knows Best.

What is the ruling on having brotherly ties between a strange man and a strange woman, and is the former considered a Mahram (unmarriageable) to the latter?

Such a relation between a strange man and a strange woman is forbidden in Islam because it involves forbidding what Allah has allowed by marriage, and allowing what Allah has forbidden such as looking and the like. Such a relation doesn`t render any act lawful between them, thus they are forbidden to look at each other, have a Khulwah (seclusion), and travel together.