الجواب وبالله التوفيق

As per the situation described in your question, according to the Islamic Law of Succession and Inheritance, distribution of an estate will only commence after funeral expenses, debts (including legal costs) and bequests (if any) made to non-heirs - which will not exceed one-third (1/3) of the Estate after debts and funeral expenses have been settled. Thereafter, all assets form part of the Nett Estate will have to be distributed according to the Islamic Law of Succession and Inheritance as per follows: 

The remaining amount of wealth should be divided into 9 equal shares; each brother will receive 2 shares whereas each sister will receive 1 share.

The 600 dollars given to the elder sister from the land which was sold in 1973, if it was given with the intention of paying off her share from the inheritance, and if the share was according to the Shar‘ai distribution then there is no need to pay her again. If the payment to her was more or less than her Shar‘ai right then she should reimburse (the other heirs) or be reimbursed (by the other heirs) respectively.

If the asset division back in the year 1973 was done according to the rules of inheritance in the Islamic Shari‘ah, then whatever amount was calculated at that time will be considered valid. Otherwise, if the division was not according to the rules of inheritance in the Islamic Shari‘ah then whenever the actual division took place correctly, the valuation of the asset will take place according to the price at that time.

(کذا فی کتب الفتاویٰ)

 واللہ اعلم  بالصواب