Sorting difficulties for a repeating group

You can’t natively sort a list by an attribute of a related thing…

This simplest way around this (usually) is to duplicate whatever field of the related thing it is onto the parent thing, to use for sorting (you just need to ensure you keep it in sync with the value on the related thing).

Alternatively, take a look at the Floppy plugin, which has features allowing you to sort a list by some other list: Floppy: localStorage, List Shifter Plugin | Bubble