Is the sister entitled to claim the amounts she paid for her deceased sister's treatment, especially after learning that her husband recovered a sum from the insurance, knowing that she does not know the exact amount she paid in detail, and her husband claims to know it through the invoices?

If a person spends money that is not religiously obligatory for them, intending it as a voluntary act, they do not have the right to reclaim it, because the Prophet, peace and blessings be upon him, said: "The one who reclaims his gift is like a dog that returns to its vomit." However, if they spent with the intention of reclaiming it, they have the right to reclaim what they spent if they can prove it with evidence. If the questioner spent "without expecting anything in return," it appears she was a volunteer, and therefore she does not have the right to claim back what she spent.