Can an uncle's wife prevent sustenance or marriage, and is marriage a predetermined provision and time from a specific person, or does a person have a choice in it?

Marriage is a provision from Allah, and no woman can prevent your marriage except by His will, the Almighty. Put your trust in Allah and supplicate to Him, and strive by all means, such as prayer and respectfully offering yourself for marriage. If you fear anyone, recite the prophetic supplication: "O Allah, we place You in their throats, and we seek refuge in You from their evils." Advise this woman with that which is better.