Is the eldest son entitled to recover from the inheritance what he paid for his deceased father's medical bills, knowing that he did not stipulate this beforehand, and his father did not make a will to that effect, and he is wealthy? And are the heirs sinful if they give him the money to avoid problems and maintain kinship ties?

Resolving disputes falls under the jurisdiction of Sharia courts, not the Mufti, because the Mufti hears from only one party. If a son spent money on his father's medical treatment due to the father's poverty and inability, then this expenditure is obligatory upon the son, and he has no right to demand its repayment. However, if the expenditure was a donation, he has no right to demand its repayment, as revoking a gift is religiously prohibited. If the son claims he spent the money with the intention of being reimbursed, and the father was not in need, his claim is accepted with his oath, and he takes the amount from the inheritance before its division, as a debt. If the son has no right to reclaim the expenditure, then whoever gives him from their share of the inheritance is not sinful, but they are sinful if they give him from someone else's share.