Is it permissible for the questioner to take from the money of his second wife or the money of his orphaned nephews to pay off his debt or to spend on his first wife and children, and is it permissible for him to spend the money sent by his friends on his children and his nephews equally?

The money that comes to orphans belongs to them; it should be spent on them, and saved or invested for them. Devouring the orphan's property is a major sin. Two matters are excluded from this: mixing your money with theirs in food and drink, and a poor guardian consuming from the orphan's money with moderation (bil-ma'ruf). Jurists have differed in interpreting "consuming with moderation." It is not permissible to take from the orphans' money to pay off debts, unless you are eligible for Zakat (definition), in which case you can take from their Zakat, or from their mother's Zakat if she is wealthy.