What is the ruling on a husband communicating with his wife's orphaned sisters by phone to offer them financial assistance without his wife's knowledge?

There is no harm in giving charity to the wife's sisters, and the one who does so will be rewarded. It is important to note that they are not mahrams to the

giver of charity, so the rulings pertaining to non-mahram (definition) women apply to them, such as the prohibition of khalwa (definition) (private seclusion) and looking at them. Conversation with them should be limited to what is necessary. If a person fears falling into temptation because of them or feels an unnatural inclination, then he must stop speaking with them. Assistance can then be delivered to them through an intermediary, such as his wife or one of their mahrams.