AITA for laughing when my dad's wife's sister humiliated her and made her cry?

My dad's wife's family came to our house for dinner on Saturday. His wife and her sister ended up fighting over me (16m) and my sister (14f). Her sister was calling my dad's wife delusional and mentioned that she's been married 3 times to 3 men who lost their wives and had kids. She said she was trying to live out her mommy fantasy that she can't have with bio kids. But she could never accept or see that none of her stepkids gave a fuck about her and that she only left her last husband when one of his kids pushed her down the stairs. And she always stayed longer and pretended she was a mom. She said it's clear to everyone except for her that me and my sister don't want anything to do with her. And that she's holding onto a marriage where she'll be miserable when she realizes the two of us won't ever let her be our mom. Her sister also said the only difference between us and the old stepkids was the fact we didn't do everything we could to make her life hell to get rid of her. But we looked repulsed whenever she tried to hug us or wanted to spend time with us. She told her to open her fucking eyes and get therapy.

My dad's wife got upset and she said it was so mean and why was she humiliating her by throwing all that stuff in her face about her ex-husband's and ex-stepkids and not being able to have her own kids.

It came up because my dad's wife was talking about my sister's birthday coming up in a week and how she wanted a mom and daughter shopping trip for the two of them. Her sister rolled her eyes and it started them off.

My dad's wife was really upset but I laughed and so did my sister. It was mostly how my dad's wife denied what her sister said and made it seem like we loved her and she is a real mom. But the thing is her sister is 100% right and we don't see her as our mom or accept her as our family. Our dad loves her for real and has let her try to step all over mom's memory by trying to play mommy and inserting herself into our lives and making us celebrate her for Mother's Day and stuff. But we don't accept it and we won't ever appreciate her or be glad she's here and if they divorce or dad dies we won't even look at her again. I don't even care if she's doing it to try and make everyone happy. I hate that she won't accept no and that we tell her we don't want another mom and she does it because we "need a mom" and crap like that.

When me and my sister laughed it made things worse. Her sister was like see and we were asked to speak up and we said she's right. I said I didn't know all the stuff about before but it was gross to try and replace dead mom's because she couldn't have her own kids.

My dad wasn't happy with us and he ended up kicking his wife's sister out. Yesterday he talked to us after he calmed down and said it was really mean to let our mom be treated like that and I told him she won't ever be our mom and her sister was right that she's delusional that she can't see we don't love or want her. He said we might feel like that but laughing was cruel when she was so upset.

AITA?