My father (who I wasn't ever REALLY close with, though we were working on it) passed away in June, and my brother will be walking me down the aisle. There's a lot of family drama in the past, and I wouldn't feel right with my stepfather or mom walking me, and my stepmom doing it would up the drama quotient beyond manageable, and I don't want to walk alone.
So that's decided, but we're thinking on parent dances now.
FH wants to do the mother/son dance, because he's convinced his mom is really looking forward to it, as long as it's okay with me. And I'm convincing myself I'm fine with that. (And I am... I just know it'll be hard watching him and thinking about my dad.) FMIL has some reservations, because she thinks it'll seem strange with a mother/son dance and no father/daughter, but she'll do it if FH wants to.
As with the aisle, dancing with my stepfather wouldn't feel right, though I'm sure I'll dance with him later... just not in the spotlight. I'm leaning toward just doing the mother/son dance and leaving it there.
Asking your brother to walk you down the aisle is perfectly OK. The decision of who to ask is the bride's personal decision, hers and hers alone. You're fine there.
Similarly, the decision about the dance is yours alone. I suggest you consider dancing with your brother, as he "filled in" for your father earlier in the day. Another idea might be to dance with your new FIL.
Bride and groom dance together for the First Dance.
Groom dances with his mother.
Bride dances with groom's father.
Then, switch around to include everyone else: Brother, stepfather, your mom, grandparents, etc etc.
Whenever a bridal "tradition" does not fit your particular situation, feel free to skip it or to change it to suit yourself. For example, you could dance with your Mom, or do a dance with your bridesmaids (obviously, your song choice might be different if you're dancing with "the girls"). If you don't want to be in the spotlight, then either skip it or do it without making a big deal out of it. The same is true for your FH. If his Mom feels weird about being in the spotlight under the circumstances, then there's nothing to stop him from quietly taking the floor with Mom while others are dancing. The dance still will be special to them, and that's what counts.