Ask your mom if she’s just trying to get you turned on to seduce you

You were already hard from her close cuddling and kisses. When your mom asks if you would spank her, your cock throbs. You take a long, slow breath before you turn to look her in the eyes. “Mom… you saw me watching that video and I told you that I’m intrigued by the idea of a Dom/sub relationship. I have to ask… do you really feel a need to be punished that way? You’re an adult woman. Or are you just asking for that because you think it will please me or turn me on? Are you hoping it will go further than just a spanking? Tell me the truth.”

She nods as you tell her to tell the truth. “It’s not just because I want to turn you on.” She looks down, avoiding meeting your eyes. “I do want that… want more than anything to give you what you’re craving. And part of me, deep down, does wish things could go further between us. Even though I know it’s wrong.”

She hesitates, struggling to get out what she has to say. A blush turns her cheeks pink. “But it’s more than just that. You say I'm an adult woman, but I… I don’t feel like I can make my own decisions rationally anymore. The feelings I have for you, and the things I have urges to do… they make me question if I’m fit to make my own decisions, much less look after our family. When I think about it, I don’t want to anymore. I know in my heart that you’re much more fit to take charge, to be the man of the house. If you disciplined me, that would… make it real for me. I don’t know why. I just know that I’d feel much more comfortable, once… once you demonstrate your authority.”

You know it must be the spell making her feel this way, especially the part about her wanting you to take charge of the family. Your mother should know as well as you do that if she were losing her faculties, Abbie would be the natural choice to take on that responsibility. Your sister is older and has always been more practical and capable of facing the world than you are. If you reminded your mom of that now, though, you think it would just convince her even more that her own judgement can’t be trusted. Now that you think about it, thanks to the spell, she really can't trust her own judgement. She would do anything if she thought it would make you happy.

It's becoming apparent that the warning on the spell was right. As the subject of the spell, your mom has become your responsibility. She’s floundering because can’t understand what’s happening to her. You’re going to have to make decisions for her.