Make sure you tell your kids THE TRUTH. DO not lie to them. And tell your brothers immediately. Sob if you feel like it, you have just been battered shitless emotionally, you are allowed ot cry.
This is the worst piece of male serving advice I see regularly trotted out to women - don't tell the kids the facts.
It is abusive. To you and to them.
You have done NOTHING wrong. Your ex husband is a lying sack of shit. He's low empathy, totally selfish and cannot be trusted. By anyone, including your kids.
Imagine not warning your children about another lying scumbag who you knew to be a lying scumbag. How dangerous and stupid that would be. Your ex scumbag husband is a danger to them and to you.
The usual "all women should be martyrs with no feelings and are entitled to nothing" suspects will disagree.
Into the future, your lying scumbag of a husband will lie about you to them. In fact, he already has been. He will have been monstering you slyly and secretly behind your back. Soon, this will be all your fault. He has already lied about you to the woman he has been fucking and others too.
He's a creepy, low empathy, untrustworthy piece of shit. Your kids deserve to know that.
Not suggesting you put it in those terms, of course. But you MUST TELL THEM THE TRUTH. It is vital for You going forwards and for them too. You are NOT protecting them by lying to them, you are confusing them and helping their lying shithead of creepy bastard father to confuse and lie to them further. Your creepy ex will very very definitely make sure he paints himself as a saint and you as a vile woman who forced him to stick his penis in other people.
Have the conversation away from your filthy scumbag ex. YOU get to tell them. YOU get to chose how that goes. YOU get to control the narrative, not your creepy, abusive scumbag ex.
YOU get to act and feel sad about it. YOU are the victim here, not the scumbag ex. And yes YOU deserve your children's sympathy and support.
Tell them quietly that you have just found out that their father has been lying to you all for years and has decided to break up the family and leave for another woman. Tell them you were blindsided, had no idea, but will try to keep their lives as stable as you can. Tell them you are not sure what to expect as he has spent all your money too.
If he has actually said he does not like your kids then that is something to tell them at a later date maybe. No normal person says this or thinks it. I think they need the warning that he is a piece of shit they cannot trust, but I can see that might be a comment better not shared.
Don't be the doormat secret keeper for your abusive shithead of an ex. He will wipe shit on your face if you do.
He is a liar, he is not your friend, he is abusive. Find your anger and accept these facts.
Briefly message anyone you choose with the simple facts, but expect no support of any kind from his family. Do not engage in his family's attempts to blame you and minimise (which will come) say nothing at all to any of their responses unless fully supportive of you as you are the victim in this. Let your silence be their conscience. Block anybody from your life that tries to defend him or minimise his abuse.
Next stop is Chump Lady dot com.
He could have left at any time before he chose to stick his penis in other women. And it's not just one, of course. Nothing he says now can be trusted about anything.
And for the love of God, get a lawyer.