Am I sinful if I use gifts given to me by a foreign sister who recently converted to Islam, given my concern that the source of her money might be from work involving alcohol due to financial hardship?

There is no objection to accepting and using gifts from a Muslim sister, even if her earnings before her conversion to Islam were from unlawful means. What she acquired before repentance is lawful for her, based on the Almighty's saying: (Say to those who disbelieve, if they cease, what has passed will be forgiven them), and (So whoever receives an admonition from his Lord and ceases, then what has passed is for him). Furthermore, it is not necessary that the money be from unlawful earnings unless there is certainty of that.