it sounds a bit counterintuitive to take something acidic like acv or lemon juice for an overly acidic stomach, but it actually does help. i have my whole like had a very sensitive and acidic stomach, even as a child.
after i started doing large amounts of lemon juice and acv the problem gradually subsided. its not an overnight fix, it took a few weeks to really kick in for me.
one of the reasons i think it works is that the lemon juice and the malic acid in the acv help to decongest the liver. i think a lot of overly acidic stomach is the result of an overtaxed and toxic liver. so i think your sister might also think about doing a few liver flushes.