What is the ruling on one who stipulates a fee for her service to her sisters, and does this stipulation nullify her reward?

It is part of a servant's understanding (fiqh) that acts of obedience should be purely for the sake of Allah, without any desire for gain or personal gratification, whether material or moral, as Allah Almighty said: (And they feed food, in spite of love for it, to the needy, the orphan, and the captive, [saying], "We feed you only for the countenance of Allah. We wish not from you reward or gratitude.") And as Aisha (may Allah be pleased with her) used to do, when she would respond to a supplication with a supplication, so that her reward would remain with her.

It was incumbent upon the questioners to involve their sister in charitable acts. If they did not do so, then there is no blame upon the sister for stipulating a benefit in return for transporting them, even though it would be more preferable for her to do so seeking the countenance of Allah, and to strive to attend the lessons with them to gain both rewards: assisting her sisters and obtaining the benefit. If her circumstances do not permit and she needs assistance, then it should be offered to her, and there is no blame upon her for requesting it.