i'm hosting several people who can't have alcohol, and somehow my best recipes all use it--i'm substituting stock for white wine, but am defeated when it comes to port and brandy
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i might try balsamic or sherry vinegar, depending on the way it's used, but i'd definitely use a lot less than however much port/brandy is called for and taste as you go... could also probably just omit them entirely?
you can always leave out the alcohol and substitute stock or water. You might think about grape juice or cider if you think it might need extra flavor. I also like using mushroom stock or veal stock in meat dishes to give more flavor.