Am I sinful for accepting gifts from my sister or eating her food, given that she lives with a man without marriage, out of fear of severing family ties, and knowing that I need these gifts if I were to donate them as charity?

The suitor should be investigated by trustworthy individuals, and if his righteousness becomes apparent, then seek guidance from Allah the Almighty through Istikhara. As for the questioner's sister, if what is mentioned about her having an illicit relationship with a man who intimately associates with her as spouses do is true, then they are both fornicators. This is because a lawful marriage has conditions, such as the guardian's permission and the presence of witnesses. The father must denounce her actions, instill fear of Allah in her, and prevent her from this evil, otherwise, he would be a cuckold (dayyūth). If shunning her would deter her, then she should be shunned. If she acquired wealth from this relationship, it is ill-gotten gain (haram) and may not be used except out of necessity, with its specific conditions. This wealth must be disposed of by spending it on the poor and needy. A poor woman may take from it what suffices her need.