Is it permissible to take money from one's sister—without being certain of her intention, whether it is a charity (sadaqah) or a gift (hadiyah)—and donate a portion of it to an orphanage as an ongoing charity (sadaqah jariyah)? And will the reward be exclusively for the donor, or will the sister share in it?

It is permissible to take the amount, according to the hadith: "If anything comes to you from this wealth, and you are not avaricious nor a beggar, then take it; and if not, do not let your soul follow it." As for its disposal, if it is a conditional gift, then the condition of the giver must be observed if it is permissible. If it is charity, then you may dispose of it as you wish, if it is lawful, and it is better to start with yourself and your family if they are in need, then give charity from it.