Would the questioner be sinful if she were to reduce the monthly amount she sends to her parents for the purpose of saving, knowing that the amount she will send will be sufficient for them, and fearing that God might punish her for that, especially since her siblings do not help her in spending on the parents?

There is no sin upon the questioner if she reduces the amount she sends to her family. Rather, she is doing good to them as long as the amount suffices them according to what is customary. However, she might be harming her son if she deprives him of his needs because of this, especially if her family is capable of earning but is negligent in doing so. She ought to rearrange her financial affairs to combine benefiting her own family and doing good to her parents without enticing them to idleness.