Secret half sibling

Fireflyaway

I could understand if you found out a secret about someone such as they had a criminal conviction, had been declared bankrupt or lost a job etc and didn't want to disclose it. However, keeping this secret is different, it's hiding the existence of a person and that doesn't seem right.  If your mum was certain she didn't want anyone to know, she wouldn't have told you. She obviously felt pressure holding the secret and now it's been passed to you. You can't control how your family will react but I think they have the right to know about the situation. I agree with others that it will probably come out some way of other in the future. My nan was very unwell with cancer and on a lot of medicine that made her confused / high! The nurse asked if the pain was ok and my nan replied that she was ok. She had given birth to 5 kids so this was fine in comparison..... My dad and his 3 sisters equals 4. Not 5. At the funeral someone came forward and explained that my nan actually did have 5 kids. The first child was hidden away and raised by a family member. In my opinion the longer you leave it the more potential damage. It wasn't your choice to be given this secret and holding it longer than needed probably isn't healthy for you and might compromise the relationship with your siblings.

Robisere

Regarding the hypothetical situation of a will made by your dad: I have two children that I have not seen since they were 5 and 7. In my will I have made reference to them by stating that my stepchildren are my heirs and I do not acknowledge any other descendants. To make this legal, the former children must be named. As I have had no contact with my biological children since 1975 and they live in another country, I do not think they will be aware of my eventual death and be able to challenge the will, but I have taken steps to protect the ones I love who also love me back.

LunaMoon15

Well everyone, there will be no spilling of the beans yet, that's for sure. 
I'm now not on speaking terms with my dad. We are very different when it comes to many things, including politics, ethics etc. He likes to argue, last week or so he called me to basically do that. I'm not sure what he was trying to achieve, but it almost feels like he did it to justify his feelings of contempt toward me. But I hung up, I had a lot on my plate and the last thing I needed that day was to be arguing with him.
Anyway, today I called him. He just went in and started again where we had left the last conversation. I gave him nothing to use against me. But the thing I'm most annoyed about is he lied, he lied about why we fell out, he lied about my late mum and lied about something my brother is supposed to have said to him. On reflection it appears he's done all this to justify his feelings towards me.
Afterwards, I momentarily wanted to phone him back and say I know about his secret! But I didn't. When I spoke to my brother later to clarify what dad had said to me, which did turn out a lie, I wanted to tell my brother the secret. But I didn't. I won't do it in these circumstances because it'll be a knee jerk reaction.
So, given the current situation, I won't be telling anyone, because it will not be talked through and dealt with responsibility. 
But, thank you, each and everyone for your advice. It has given me food for thought. But now is definitely not the time. 

Unicorn_cottage

@LunaMoon15 - nothing to add but to send you a hug, What a difficult situation. I wish you well for the future. 

unholyangel

This is way more common than some realise. Sometimes, it will only come out when they've passed away - particularly here in scotland with children's legal rights! 

You know your dad best. And you'll probably also have an idea of whether your mum's warnings (not to say) were well placed or not. 

I'd normally say it's not your secret to tell but I think in these circumstances it's one of those it's difficult to tell the right thing, particularly as people have different views on family. Most people place importance on sharing DNA to a degree - even if it's between parent & child. If you're of that view then it might lead you to think the kids have a right to know they have other family. Some people don't though - they consider adopted/foster/step family as family or make their own family from those around them and want nothing to do with the people they share DNA with. 

Just remember that you can't put the cat back in once you let it out.