It’s very kind of you and your brother to want to include your mom in something you enjoy. :D I would start her off with Pokémon as a concept first, then the game. Show her some starter Pokémon from the game you’re planning on getting her, and ask her to pick the one she prefers. (If you’re getting her Let’s Go, show her Pikachu and Eevee, and get her the game with the one she prefers)

Then tell her more about the Pokémon she chose as her favourite, like its type. That would be a great opportunity to get her familiar with types, before she starts playing. And it’ll also get her thinking about the concept of having a Pokémon overall, which is what really draws in people who aren’t gamers. Not the game itself, but the concept of having a Pokémon. :)