How to correctly capitalize words inside of brackets/parentheses in article names and book titles?

How do I correctly capitalize words inside of brackets/parentheses in names and titles?

A few examples:

Captain America (untold stories)
Captain America (Untold Stories)

Madonna (written by E. Cales in 1998)
Madonna (Written by E. Cales in 1998)

My Summer Vacation (with my family)
My Summer Vacation (With My Family)
My Summer Vacation (With my family)

It depends on what you're qualifying as parts of the title vs subtext, with consideration to important words. For the examples you gave, I would use:

Captain America (Untold Stories) because "Untold Stories" appears to be a part of the title, and both of those words are significant.

Madonna (written by E. Cales in 1998) because the 'written by' is not part of the title.

My Summer Vacation (With my Family) could be argued as (With My Family) - essentially, the "my" in this case is acting as a bridge - like "the" or "of" in Phantom of the Opera - they are not significant words, but they are necessary.

Expanding on Helpful's helpful answer - I would use document formatting as well if possible to differentiate titles from other info, e.g.:

Captain America (Untold Stories)

Madonna (written by E. Cales in 1998)

My Summer Vacation (With My Family)

Or, add quotation marks:

"Captain America (Untold Stories)"

"Madonna" (written by E. Cales in 1998)

"My Summer Vacation (With My Family)"