I found out my dad...

Tell your father what you found. Throw the ball back into his court; this is something for him to deal with, not you.

While you shouldn't have gone snooping after you saw the first email (you should have come to him then), what's done is done. If he opened up his email to you, then he had to have known it was possible that you would find him out.

Here's your script.
And see what he says. He might beg you not to tell your mother. In that case, you've made it clear; you're not going to lie although you might slip up. It also makes it clear that there's a possibility that you might err and share this information with your sibling.

He might claim he's already told her. Then in that case, you can say that you might mention it to her on occasion but you won't be bringing it up as dinnertime conversation. In that way, if he's told her, then fine. And if his claim is a false one, and he hasn't, then he will if he's got any sense.

Ending your talk with concern for his health and affirming your love for him is important. His secret is out, kinda, sorta. You need to assure him that he's still your father and that you still respect and care for him. But you are also mindful of your mother's future health (and his as well).

This little speech should cover that. Ultimately, this is between your parents, and you should never have been made a part of it. But you are right now. They may not see it as cheating; they may have an open marriage and just see this as a part of it. You don't know what really goes on in their marriage, despite your "best friend" status with both of them.

Do your best not to judge their choices, okay?

Unless there is conflict or unhappiness in the marriage for your mother I would not mention it.

Most likely she may already know about your father’s affairs.

I gotta agree with Punkey. Their marriage is their business. Your mom may know or she may not. But I don't see any good coming out of your inserting yourself into the issue.

I agree with Jespah. Tell your dad what you found out, and ask him if she knows. If she does, great, no harm no foul. If she doesn't, then you should probably advise him to tell her, and point out that if you found out, she could find out too, and that the potential backlash might be more significant if your mother found out on her own.

Apart from that, I wouldn't tell her yourself: it's not your secret to tell.

This assumption SHOULD NEVER BE MADE. Just ask Alison Bechdel, whose father committed suicide because of his hidden second life as a gay man came out. Something like this maybe known without really actually being known by the spouse.

The father's outside marriage relations need to be addressed as the marriage probably isn't the healthiest. Extra marital affairs aren't healthy (even if they're buried like the proverbial ostrich head in the sand (which in itself is an urban legend).

I'm still trying to figure out what reason there was for going through the emails in the first place. Did he even know you were planning on it, or, is this just something you took upon yourself and it's a blatant invasion of privacy?

You did't need to do anything, just moving on. Your dad loves his family so much and you did't need to interfair his personal interests. If you do, it will bad for your family love and bounding.

You need to care about your family relations before taking any decision

I can only go by what I would want.

I would want to know if someone I was intimately sharing a bed with was having affairs.

I would cease all sexual contact at that point and demand a full STD testing.

My health is more important than their sexual conquests.

I tend to think that asking him if your mother knows would be the first step, second step is to expect him to tell her if she is unaware before he engages in any further sexual contact.

Emails and cyber sex is harmless (sort of) but actual physical contact is a serious violation of trust. If she does not know then you cannot really trust the extent of what he says has occurred either. If he will deceive her, he will deceive you.

If he says she knows about it then there would be no reason why you cannot discuss the issue with her.

If she does not know then you probably have a duty to see that she is somehow informed.

I would find it very hateful if a "friend" or sibling kept me in the dark about something as serious as this.

Be aware that many wives look the other way regarding this issue. Your mother might resent you for bringing the topic out in the open but at least you have not been the one who has deceived her.

I would rather be resented for exposing the truth than be part of the deception.

This is just my personal opinion as a gay man, a lot of gay men expect fidelity too.

Be prepared because the "truth" might have far reaching consequences.

Maybe this man, your dad, is not your real father and he stayed around to raise you.

I am just saying this because stranger things have been known to happen.

It is not likely that he is not your real father, I would think you would know through family resemblance and such.

Some secrets have a way of unraveling more than one might expect.

I hope this is not true and it is just a case of a straying husband.

I am saying this only to prepare you for the worst case scenario...