I can imagine if Sultan got everyone together and told them he's getting married everyone would act excited. Everyone except Jasmine, I think it would be like this when she started talking.

Jasmine: How could you?
Sultan: Jasmine wait! I love her and I wanna spend the rest of my life with her and she cares about you she.....
Jasmine: SHE'S NOT MY MOM AND SHE'LL NEVER BE MY MOM!
Aladdin: Jasmine calm down and let him try to make you understand.
Jasmine: Fine! Explain this to me father! How can you think that there's anyone in this whole world who could ever replace my mom?

It would definitely be interesting

Jasmine will hate every inch of her new mother.

I could definitely see her being pissed about this.

Another teenage problem to deal with.....Ariel has enough problems that she has caused

She would be very angry. Also Ariel.

I don't think Belle would have a problem nor Pocahontas (of course, if the stepmother is nice)

I don't think Pocahontas or Belle would be upset but Ariel would not as much as Jasmine. I imagine her, Eric, and Melody being invited to dinner at atlantica and Triton tells them all about it. Ariel is kinda shocked and goes to get freash air(or freash water) and think about things. Melody and Eric try to see if she's okay. She tells them she's fine just a little shocked and starts talking about her mom. Than Triton comes and ask Melody and Eric to leave him alone to talk with his daughter.

Triton: I guess the news of the engagement is kinda a shock huh
Ariel: Kinda
Triton: She's very nice Ariel I think you'd like her if you met her
Ariel: I know I will because you have great taste in women, after all you did a good job the first time
Triton: Yeah you're right I did, but you seem a bit upset
Ariel: It's just that I can't see you with anyone else but mom
Triton: I use to think the same thing, what me and your mom had was special nobody would ever take that away
Ariel: Than why are you engaged
Triton: Because we all need to move on, your sisters didn't take it too well either but they got over it
Ariel: Why did you wait this long to tell me
Triton: Because I knew out of all of you girls it be hardest for you because you were the closest to her
Ariel: Yeah I miss her so much
Triton: I do too but whenever I missed her I just looked at you, when I look at you I see your mother. I see her smile, nose, chin, hair, and that forehead I love to kiss.*kisses forehead*
Ariel: Really? Well daddy if you really love this mermaid than I give you my blessing *hugs Triton*

You can tell in the 3rd movie how much she loved her mom. They all did of course, but we actually see how everyone was very sad when her mom died. I think Ariel would hate to get a step mom.

or ariel

Ariel or Jasmine, either one.