My dad married a new woman without telling my mom, should I pressure him to tell her?

Asalamu alaykum. My father remarried and he didn’t tell my mom, should I keep his secret or tell my mom?

This problem started when my mom and siblings moved to Canada to work here. My father didn’t want to come here, he wanted to stay in a Muslim country. My mom provides more for the family, and my siblings study here. I wanted to visit him last year, and he told me had a secret I should not tell anyone after I visited. He showed me the ring before I went, and I was very confused and didn’t know what to do. When I went there I was very uncomfortable with the new wife and thought he should tell my mom, so I told him this.

My mom was Christian, but she converted, even though she doesn’t pray very much. His new wife is likely ahl al kitab and not Muslim. He told he married another woman because he was lonely. He also said he will tell my mom, but that now wasn’t the time because my mom is dealing with a lot. Since then, he’s lost his job there, and he’s unsure about his new relationship. He also visited us here, but didn’t say anything or tell my mom. He told me he’s trying to mend the relationship, so I’m not sure if he will even tell her after he does that. His relationship with my siblings is not good too.

I know in Islam he doesn’t require permission from his first wife, but the situation seems unethical. I feel I should tell my mom because it influences her life a lot, and I believe she should know the full situation to make her decision. But I also know in Islam that we should try to keep family ties secure, and not break them, regardless of the situation.

I am very lost and confused, and this situation has caused me a lot of trouble. I don’t want to be the one to break up my family, and I don’t want that to happen because I don’t think my siblings or my mom would like him after this. I know personally, he is still my father so I will still treat him with respect, etc. But I don’t think what he did is right. I think he made a mistake by not moving here with his family, but maybe he was staying there as a financial backup in case my mom didn’t find a job. But even when she did, he stayed there. I think he also made a mistake by marrying another woman when he didn’t need to, especially without telling my mom. It seems like he didn’t care what she thought, or he never planned to tell her, or he was having a backup in case his relationship with my mom didn’t work.

I don’t know if I should tell my mom because this would be breaking my promise, and hurting my family relationship.  But I feel bad for her because she doesn’t know the full situation, and if he remends the relationship and she later finds out, then this will just be worse for everyone. I asked my friends and they said it should come from him and that I should stay out of it. But I feel like I’m lying to my mom by doing this. I don’t know what he’s expecting, and I don’t know what to do. From an Islamic perspective, should I tell her and break my promise and likely my family? Or should I keep his secret, and basically lie to my mother? He should have hid this from me because it’s hurting me a lot to keep this from my mom, while I talk to her almost every day, and joke and smile, while knowing this painful secret. I’m also worried for my family because they are moving away from Islam, but I don’t think he would even help with that.

Sorry for the very long question, but I hope I can get an answer on what is right to do in this situation.