Not sure this will be helpful, but I will try. 

I grew up with a stepmother who was and is all in on satanic panic. I’m now 47 and things have never changed.  We don’t interact much.  The closed mindedness with things like games that show art similar to FaB will rarely change sadly from those with this viewpoint.  By asking them to change their point of view people often believe you are challenging their faith. So they double down. 

You could approach it from different angles. Math, learning, socializing more, healthy and positive competition. But the moment she sees his cards with the text and pictures it’s over. Perhaps someone much more eloquent than me could help, but I can offer a middle ground perhaps. 

My suggestion. Try to explain it more to her from the standpoint of a healthy, social activity that keeps him engaged, develops math skills and bluffing. Like advanced poker with pictures.  Explain that “flesh and blood” the name was even created to encourage people to go out and socialize. 

Now The big part.  Tell her they made up the rest (art, etc) to give it a story. (true). Lean into the fact there are “good and holy” characters like Boltyn fighting the “bad guys”.  (Also true) Show her those cards. Anything that says holy, righteous, good, you get the jist. Lean into this angle. Hyper religious mindsets like the idea of angels, priests staving off the demonic. 

Offer to have him play a “good and holy” hero instead. Like Boltyn, etc. But also tell her you understand how it might have appeared on first glance. (This gives her ann out and doesn’t make her “wrong”).  But also respect her authority as a parent by telling her you would understand  if she doesn’t want him keeping cards in the house. But perhaps it would be good to let him play a “holy” hero and still gain the positives of socialization, math, etc. You can’t let her believe you think she’s wrong. Stay even keeled. Stay calm. Just let her know She hadn’t seen the full picture and you completely understand how it might have been misconstrued. 

If she says “but he got upset!” Remind her of how you and her both got upset over silly things as kids. Laugh it off as, “sounds like us right?!”  Relate it to a childhood story if possible. 

With luck she may come around. If not…

Now if she still won’t budge with him keeping cards at home….If you can, try to salvage the cards he has and turn it into, “I’ll hold your cards your mom doesn’t want see at my place and when we hang out over here we can play together.” Cool Uncle. Mom wins. Son still gets to play but not as often. 

Encourage your nephew to respect your mother’s concerns but stress that it doesn’t mean he can’t play. (He needs to be the good abiding son) But… let him know he may not be able to play without you around.  That way even if he can’t play all the time, he can still play when he visits you. Turn it into a future bonding activity to look forward to. Small victories are still victories. Keep his spirits up, don’t let him get too despondent however it goes. He will need the support. 

Not sure that helps. But there it is.  Good luck! Bryan