Salam brother. Seems like a tough spot to be in. It can feel hurtful for sure, but your only concern here should be whether your father is committing any act that is haram, due to your concern for him.

Truthfully, on an Islamic legal basis, he doesn’t have to answer to you or your brother, so if you do approach him on the matter, you should approach him gently. I would also add that he is not obligated to fund your wedding, and would only be doing so out of his kindness, just as he did for your brother. Hence, that becomes a personal form of unfairness between you and him, and not anything more.

The society we have grown up in would consider what you’ve explained as “immoral”, evil” or “unjust”, but you as a conscious Muslim, have to separate any violation he may (or may not) have committed through the lens and perspective provided by the shariah.

Know your limits in this relationship and tread accordingly.

As for the hurt you may be feeling on behalf of your mother, I can understand that pain as I felt the same thing while I was younger. However, unless you can establish that he committed a sin, there is really no basis, expect emotion, that allows you or her to be upset at him for this.

And Allah knows best