Is it permissible for the questioner to keep his deceased sister's inheritance money to help him get married, with the intention that the reward and recompense be for his sister, given his difficult financial circumstances, and is it permissible for his siblings to help him from the money they inherited and allocated as charity for the deceased?

Your share of the inheritance is your exclusive property, and you may dispose of it in any way you see fit. If you are in need of it, it is best to spend it on yourself and your needs, and you will be rewarded for that, as the Hadith (definition) states: "Any man who earns lawful money and feeds himself and clothes himself, then clothes those below him from Allah's creation, for him there is a zakat (definition) in it." And according to the Messenger of Allah, peace and blessings be upon him: "Begin with yourself and give Sadaqah (definition) to it." As long as you are rewarded for spending on yourself, you may gift this reward to your deceased sister. It is commendable for your siblings to give Sadaqah to you, as the Prophet, peace and blessings be upon him, said: "Sadaqah given to a poor person is one Sadaqah, but to a relative, it is two: Sadaqah and maintaining family ties."