Does having a good opinion of Allah and supplicating change the provision of His servants, or is their provision predetermined and unchangeable no matter what they do?

What your sister says, that supplication (du'a) and having a good opinion of Allah are not beneficial, is false. Supplication is one of the most beneficial means, and Allah the Almighty says: "And your Lord said, 'Call upon Me; I will respond to you,'" and "And when My servants ask you concerning Me, then [answer them], 'Indeed, I am near. I respond to the invocation of the supplicant when he calls upon Me.'" Furthermore, having a good opinion of Allah, coupled with exerting effort (taking means), has immense benefit. Continue to supplicate with certainty and trust, without hastening the outcome, for the Prophet, peace and blessings be upon him, said: "The supplication of one of you is answered as long as he does not hasten, saying, 'I supplicated, but I was not answered.'" We advise rationality in accepting a husband and prioritizing one who possesses good religion and character.