Praise be to Allaah.
There is no doubt that part of a person's understanding and eagerness for good is that his righteous deeds should be sincerely for Allah and free of any other motives such as making gains, whether it tangible or intangible, even if it is only thanks from the person he treated kindly or a duaa' that he asks for from the person for whom he did a favour.
Hence Allah, may He be glorified and exalted, said concerning His believing slaves, the people of the highest levels of Paradise (interpretation of the meaning):
And they give food, in spite of their love for it (or for the love of Him), to Miskin (the poor), the orphan, and the captive,
(Saying): We feed you seeking AllÃ¢h's Countenance only. We wish for no reward, nor thanks from you. Verily, We fear from our Lord a Day, hard and distressful, that will make the faces look horrible (from extreme dislike to it)'
It was narrated from Ubayd Abu'l-Jad that Aa'ishah said:
A sheep was given to the Messenger of Allah (blessings and peace of Allah be upon him) and he said: Divide it into portions and distribute it. Every time the servant came back, Aa'ishah said: What did they say to you? She (the servant) said: What else could they say? (They said): Baarak Allahu fikum (May Allah bless you). Aa'ishah said: And may Allah bless them; we say to them the same as they said to us, and the reward will still be saved for us.
Narrated by al-Nasaa'i in Aml al-Yawm wa'l-Layla, 303; also by Ibn al-Sunni, 277. Shaykh al-Albaani (may Allah have mercy on him) said: Its isnaad is jayyid. Takhreej al-Kalim al-Tayyib, no. 175
You and your sisters who go with you should have hasten to include your sister in this righteous act that you are doing, even if you did not owe her a favour for driving you. This is part of loving good for the Muslims and the duty of spreading good among the people.
If you did not do that and she is eager for the good that you have, there is nothing wrong with her stipulating this benefit in return for driving you, although it would have been better in her case to do that sincerely for the sake of Allah alone rather than asking you for something. She should strive to do righteous deeds and attend the lessons with you as if she is one of you. That would combine two good things, helping her sisters to do something good and seeking reward for it with Allah, as well as attaining the same benefits as you.
If her circumstances do not allow that and she needs help from you, then you should give her this help and there is nothing wrong with her asking for it from you.
And Allah knows best.