If you are using "Aunty Mary" as a name (Good morning, Aunt Mary. I went with Uncle Bill and Aunt Mary. -- The same way you'd say "Good morning, Frank" or "I went with Peter and Alice") then use the capitals.
If you are referring to her like this: "I went with my aunt, Mary Smith, and two of my cousins" then it's a generic noun and not part of her title.
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I had the same question. In a sentence: My aunt Jane is very nice. Does "aunt" have to be capitalized?
my aunt Mary
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