I don't know who you might invite, but there is a long lost tradition of a "memorial meal" where you and others gather to share a meal and remember the person you love. I'd say loved, but you still love her. It's a way to celebrate her life among people who held her dear.
As for the stepmom thing, that will always be weird. I dearly love my stepmother, but occasional weirdnesses still pop up. Hallmark doesn't make a "You're a really nice lady, and I'm glad you make my dad happy, but you're not my mom, so happy Mother's Day??" card.
They do make a generic one that doesn't use any variation of the word "mom." If it's not too hard, give her a card focusing on the fact that you're happy she's with your dad.
Good luck, family stuff can be hard to navigate.
As for the stepmom thing, that will always be weird. I dearly love my stepmother, but occasional weirdnesses still pop up. Hallmark doesn't make a "You're a really nice lady, and I'm glad you make my dad happy, but you're not my mom, so happy Mother's Day??" card.
They do make a generic one that doesn't use any variation of the word "mom." If it's not too hard, give her a card focusing on the fact that you're happy she's with your dad.
Good luck, family stuff can be hard to navigate.