Is it permissible to demand a portion of what was spent from the inheritance – after thirty years – on the father's increased religiosity and the sisters' marriages? And how can the value of the amount spent (50,0 pounds) be estimated after this long period, taking into account the change in gold prices and real estate values?

If you have repaid your father's debt or spent money on marrying off your sisters as a donation, without intending to reclaim it, then you cannot demand it back. This is because retracting a gift is forbidden by Islamic law, especially when given to relatives. However, if you spent the money with the intention of reclaiming it, then it is a debt that you are entitled to demand. If they do not believe you, then you can either seek your reward from Allah or take the matter to the Sharia court. If your right is established, you are entitled to claim an equivalent amount, unless there is excessive exploitation or currency collapse.