Dhampir have two types tags in the new Unearth Arcana, they are both humanoid and undead. There is even a nice rules reminder in D&D Beyond that points out:
If an effect works on at least one of a creature’s types, that effect can work on that creature. For example, if you are both a Humanoid and an Undead, cure wounds works on you, since the spell works on a Humanoid.
Everything so far is peachy keen. I was going to introduce an NPC that is a Dhampir Cleric, but ran across an odd edge case. If the cleric uses Channel Divinity: Turn Undead I'm not sure what happens:
Channel Divinity: Turn Undead
As an action, you present your holy symbol and speak a prayer censuring the undead. Each undead that can see or hear you within 30 feet of you must make a WIS saving throw (DC 14). If the creature fails its saving throw, it is turned for 1 minute or until it takes any damage. A turned creature must spend its turns trying to move as far away from you as it can, and it can’t willingly move to a space within 30 feet of you. It also can’t take reactions. For its action, it can use only the Dash action or try to escape from an effect that prevents it from moving. If there’s nowhere to move, the creature can use the Dodge action.
I know it is play test material and will have weird edge cases like this not hammered out. I also know at my table I will have to make a call if it comes up. However, as it stands, the NPC is an undead within 30 feet who can see and hear itself, so RAW would have to make the roll, right? And what would it mean if you failed it and became turned and had to move away from itself? What would that look like?
Best Answer
The simplest answer is if it's fun, keep it in! And I mean fun for everyone. Fun might not be "silly haha jokes", to some it's the consistency and security of the rules that makes things fun so something like this may upset them. If not, keep it in!
As for reasoning it all depends on the mechanics of your world. It's up to you how you wish to flavour it, but you could argue that if they're drawing this from a domain due to their connection to it/the gods, that their connection is negating this effect. In other words their source is either passively, or actively, deciding not to turn them as they are an effective agent. Kinda like how some of the raven queen balances life and death, and hates the undead, yet stops those who serve her from passing on. She's allowing them this ability to perform your duties.
Perhaps this Dhampir is allowed this effect without it harming them because of their devotion? Alternatively, maybe channel divinity is summoned from inside the characters, and is only a tangible effect when it's expelled from their body, like how a flamethrower doesn't burn the one using it as the energy is only turned into fire when it's expelled outwards.
There's tonnes of ways to flavour it, you could even have it as a mystery for the party to solve! Maybe ask them/yourself why channel divinity turns creatures away in the first place? Is it the gods/domain going "Nope, you're bad, go away"? If so, does devotion mean you're showing that you're a servant to that domain? That you're not bad?
These abilities are meant for combat and to give the players a tool against enemies. Maybe at the very least let OTHERS be able to turn undead against THEM, but if it's the Dhampir cleric casting it? I can see that mechanically making sense.
Balance the fun, get feedback from the players, and just go for it! If it's a terrible idea have a rock fall on them, but I'd bet money that your players will appreciate your creativity and love that you tried.
Also the original games are very problematic from their source material. I mean, Phylactery? Goblins with big noses stealing money? You mean a device meant for carrying around small scripts of hebrew text, and a stereotype of Jewish people? Have fun, change what needs to be, and keep the good stuff! You'll do great!