Nth-child value saving on user interaction; transferring to other elements with nth-child values?

I have run into a bit of a quandary with the macro I want to write…

Q: Is it possible to keep track of the value of an nth-child(n) and to have a string or multiple strings that can give the saved value of the first to another nth-child?

For instance:
Being able to click play and get the nth-child value from that click > live feed the value from the first nth_child into the other nth-child values that in this case puts the playing track(first value) in a playlist(this sequence consists of 2 clicks on 2 different elements) as to have the value from nth-child 1 be brought over to nth-child 2 & 3.

so it has to check when I click on play
?better to have it check at all times or have have it check whenever I click?
so the check has to find the (n) in li:nth-child(n)
as to then feed (n) to the playlist sequence which is 2 commands and have it go to those adjecent 2 li:nth-child(n?)

I hope any of this made sense; is possible. I have been thinking about using .index or executeScript_Sandbox but I am having a hard time using this and applying it.

Do you have an example page for me to look at?