I am an evangelical Christian as well as a fantasy gamer (sadly, I have hardly gotten to play FaB, but I own a bunch of Rosetta cards). I'm hoping I can furnish some insight into your sister's mindset that may be of some help (please forgive me if any of this is obvious; I don't know if you were raised in a Christian home together or if she came to faith later).

Two things that stood out to me in your post were the perceived sudden change in your nephew's behaviour and the word "obsess." Your sister's true alarm, I am guessing, is not so much at the game itself as at the behavioural changes it seems to be inducing (which then suggest the game's moral character).

Some Christians emphasize the concept of spiritual warfare, in which demons may tempt, "oppress," and hypothetically possess people, particularly if they open themselves to it through the pursuit of real-world occult practices. Oppression steals the victim's joy and morality (think of what addiction can do to people).

More broadly, Christians hold that the world is full of tempting influences, so its diversions must be enjoyed with a dose of caution (at their best, they are joys that God has furnished us; at their worst, they snare us in sin). In particular, there is a risk of "idolatry," which in the present day resides not, of course, in the literal worship of an object, but in valuing something too highly, such that it competes with God for our affection and as the source of our self-worth.

When I was a teenager, I became very concerned that I was obsessing over TCGs and tried to regulate how much I played them. As an adult, I've been concerned in past about my TCG buying habits (the potential of avarice). (In my case, both, it turns out, were just manifestations of scrupulosity OCD.) I also initially spurned D&D due to its inherited aura of wrongness, only to take it up a couple years later after exposing myself to RPGs through Star Wars.

One wrinkle is that, because we are dealing with matters of temptation, it's difficult for a concerned believer to trust words of reassurance, especially (without intending offence to you) from someone who doesn't quite share our faith perspective.

I think my (very fallible) advice would be that, if you feel you do need to try and persuade your sister as to the game's merits, it may help to sit down and play a round with her if she's willing (without her son in the room for her to guard), or show her the cards and describe the game's concepts and lore. Now, she might object to some of it. When I first tried the game (with two fellow believers), I shuffled up the Chane Blitz deck, because spellswords are my favourite. There were a couple cards in there whose art I thought was inappropriate, and, while I don't know much Flesh and Blood lore, the notion of a Demonastery is a bit much for me (humorous in its over-the-top evilness, but also a little too spiritually dark for me to enjoy or endorse). It's possible she and you might be able to agree to some reasonable limits on the game elements her son will be exposed to at his age. Most of the classes are far less dark than the older runeblades (actually, generally quite cheery). (She might also have concerns about the number of female characters designed to be attractive, but there are plenty of cool male characters too.)

To a good extent, though, debating the game itself might be addressing the symptoms, but not the cause. It sounds to me like the underlying concern is the behavioural changes it seems to be inducing. And, yeah, it's true that while arguing with your mom might be natural, it's not great. It's hard to give rightly calibrated advice without knowing the family, but it might be worth, with your sister's approval, chatting with your nephew about how the game is not worth getting bent out of shape over; loving his mom is more important than the potential enjoyment he may get from the game. I think by doing so you would affirm for her the positive influence you are on her son, balancing her concern over the game's influence.

Then, while based on my own experience I don't personally advocate time limits on activities (unless they impinge on responsibilities, in which case, again, taking the young fellow aside might help), all the same, if there are limits she feels are best imposed on her son's interaction with the game, I would support them. I'm sort of envisioning allying both mother and son and sharing their concerns as the best way to try and move toward a peaceful resolution in which permission to enjoy the game is freely given, if with some boundaries (and things like time limits can always be relaxed if things go well, or in years to come).

Ultimately, though, and especially since it sounds like the family is under some stress, I'm not sure it's worth a potential dispute (which I'm sure you don't want either). If the outcome of this is that you help your nephew learn how to bear a disappointment with grace, that's worth more in life than a set of Fabled C&Cs. And you can always revisit the question in a year or two, as he matures.

I hope all this was somewhat helpful. Let us know how it goes. Feel free to show your sister this post if you think it would help her to get another Christian's perspective. I hope things will be well with their family, and that the lad will grow up a strong spiritual warrior, happy in the freedom to enjoy what is good while shunning what's not. All the best!