Background of my Tiefling
So I made a pretty in-depth background for my warlock that intertwines deeply with the bard in the party. Their family has a long-running pact with Graz'zt and due to this, every other generation or two, a tiefling born; a 'blessing' from Graz'zt for the family's service and devotion.
We decided we wanted our characters to be twins who have a good relationship. Originally we were both making tieflings, but he decided that he wanted to make his bard a human, and I still wanted my warlock to be a tiefling, the DM didn't seem to have an issue with it and approved the characters. (Honestly, it was nice to already have them know each other and have no trust issues with half of the party)
So fast forward to when the characters meet, introduce/describe themselves (Human Bard, Tiefling Warlock, Elf Ranger, Half-elf Barbarian)
Once it is established our characters are not of the same race but are fraternal twins, the barbarian (who is an outlander from a plains tribe and has never seen tieflings before and has an INT of 7) immediately goes off about the logistics of how that can't be possible, and denies that they can be anything more than half-siblings. And then got out of character to tell us that tieflings are their own race and can't be derived from two human parents.
So here's what the PHB says on pg. 42:
"Tieflings are derived from human bloodlines, and in the broadest possible sense, they still look human. However, their infernal heritage has left a clear imprint on their appearance."
I understand that they are their own race and tiefling + tiefling = tiefling, human + tiefling can = tiefling, and in some cases devil + human = tiefling.
But especially this scenario, with a blessed (or cursed) bloodline, can human + human = tiefling?
I'm only interested in lore from D&D 5e.