Do the daughters of the waqif's (endower's) sister's son take two shares from the waqf, given that they are both the daughters of his son and the daughters of his sister, or is only one share calculated for each of them?

If the endower stipulates giving to children, both males and females, with each individual receiving one share, then the endower's stipulation must be followed. The endower's condition regarding the proportions and characteristics of the beneficiaries must be observed. So, if he stipulates equality between male and female, his condition is to be followed. In the case where your son married your sister's daughter and they had two daughters, each daughter here receives one share, not two shares. This is because it is apparent that the endower did not intend for an individual to receive two shares; rather, his intention was for each individual to receive one share. The words of the speaker are to be interpreted according to their custom and common usage. If the speaker's intent is known through evidence, then it must be followed, as words indicate the speaker's intent. If a dispute arises, reference should be made to the Sharia court.