If you want to nitpick: please don't post caps, and a comma would not help (and would, in fact, be quite wrong).

The simplest solution would be "How both my sister and my girlfriend learned to code."

That said, I think after the initial double-take the meaning of the title becomes clear and there isn't actually a need to change it :)

Where exactly would a comma go in this title that wouldn't make it worse?

ex: "my sister, and girlfriend,"

how, my sister, and gf, learned to code

You can't just throw commas around all willy-nilly.

Yes... I kind of just brainstormed the blog post, did not expect it to end up on HN

It's great that you shared this with the internets, even if you were in a hurry. Now that you have some feedback on the article you can iterate :-)