I agree with you Apple that my mom should be told. That is why I am asking the question. I just don't want to be the one to tell my mom because I'm not the one who should in the first place. I think it should be my dad or like Gwen said his Bishop if it is too hard for my dad to say it.
What I care about right now is that my mom isn't hurt because no one told her about my dad being sealed to my brother that passed away as a baby, for he has no voice here on Earth. And it would hurt my mom.
My mom and my dad were never sealed together, only with their new spouses and recently. Both my parents took a very long time to get where they are today... lots of mistakes. I'm very proud of them that they have repented and did what was right to get to be sealed to their new spouses. It was a sheer coincidence that they did it near the same time (within a month of each other) although they have nothing to do with each other anymore and haven't for 25 years.
My older brother is 40 and if he doesn't want to tell my mom, that is his choice. I'm not going to say anything, it isn't my business. But I know if he doesn't, she will be very sad that he didn't tell her to begin with.