what would you do?

What happens when he dies and the will becomes public? They will find out then the existence of a brother. The dilemma for you is that it's not really your place to tell them. Whom will they blame for being kept in the dark, especially when they realise that you were in the picture all the time? Do you speak to your ex? If you are on speaking terms, it would be right to point out to him that they will find out when dies and that it would soften the blow if he broke it to them gently sooner rather than later.

ellie50 I feel that your adult children should be told that they have a half brother. As they are close to their father, they will no doubt have mixed feelings. However, they are adult and will hopefully react in a positive way once they recover from the initial shock. I sympathise with you. I wish you and your family a satisfactory outcome.

Won't they need to know if their half-brother is included in their father's will?
They are probably old enough now to be able to cope with the knowledge and may even form a relationship with him. In general, I have found that secrets will out and being honest is the best policy. You could tell your ex that you think they should be told, and if he won't tell them, you will.

ellie what a difficult position to be in. What has stopped your ex from explaining to the children that there is another brother? He is the one who has put you all in this position, but now someone needs to explain to your children. I think they are entitled to know about this son, so they can make an informed decision about whether they want to make contact. Likewise, this brother is entitled to know. He might already know and decide one day to seek them out.

Are you in touch with your ex now, and would he be prepared to discuss the matter with you? There are ways of broaching such sensitive subjects without creating family breakdown. They might already suspect something, you just don't know until this issue is dealt with. Good luck, whatever you decide.