What would you do?

You're right, Harrigran, that's what I have done - I've told her she should discuss with her doctor. I certainly didn't reassure her that all is ok, because she has a recurring coldsore on her lip which is what her sister started with. I said she should have it checked out medically, and she poo-pooed it.
I had a mole on my arm which was getting bigger and it turned out to be pre-cancerous - it's quite common.
I've decided to stay out of it now, I only did it because she asked me specially. Just listen, and say "go to the doctor's."

If you do not have medical qualifications you are not able to sift the junk from the truth and you could either scare someone or reassure them that all is okay when it really isn't. Direct your friend to a professional.

That's an interesting link, Nelliemoser, thanks. I'll leave it for while before telling her about it. I took her the article today, she was grateful but her husband was a bit upset, went out of the room. I don't think he wants to think about it.
Another point is, it's not certain that they are identical. She just says they look alike.

Suggest she talks to her doctor.
From that article it seems that the level of risk varies greatly depending on the type of cancer and whether or not they are identical twins or there are genes that run in your family.

"BRCA1 and BRCA2 are two examples of genes that raise your cancer risk if they become altered."

www.nhs.uk/Conditions/predictive-genetic-tests-cancer/Pages/Introduction.aspx

Yes, tell her, but see if you can get someone to explain the stats to her....they are not very clear in that report.

A 43 per cent role in the risk of skin cancer does not really help.

What does that mean in terms of 'how likely am I to get this condition?'

Perhaps you can give her a helpline to call?

Thanks for all your advice.
I'm going to print out the above link and give it to her, and also suggest she goes to her doctor and see if she can have genetic testing, if she's so worried.
She said yesterday that one of her sons had told her that identical twins don't always inherit what she calls the "cancer gene". Which is in a way correct, but it seems to be much more complicated than that.

I think that you have no choice but to tell her - presumably she would not have asked if she did not want the information. Also, having acquired this knowledge, you either tell her the truth or a lie. The latter is simply not an option I feel. But, as others have pointed out, you could encourage and support her in the next step.

Oh dear, not easy for you, but I would tell my friend what I had found out and offer to support her in contacting her own GP for a check / more information. Especially as she asked you, she must be carrying some anxiety and she may actually feel better once the subject has been properly aired with medical back up.

I think if the family has a history of cancer it would be a good idea to seek genetic tests and counselling. The woman concerned can then be re-assured if her cancer risk is normal and have regular monitoring if there are concerns.

Stomach cancer runs in DDiL's family. Her DF died of it when she was only 5. She has had screening and been told that her risk is only slightly above average. No monitoring is required but she has been told to go back to her doctor should she have any symptoms that could be of concern.

I would tell her what you found and then suggest she sees her GP for advice. Skin cancer is quite common and people have melanomas removed under local anesthetic these days. The tissue is analysed and things are taken from there.

Does she have anything on her skin she's worried about? Her sister's skin cancer might be down to spending more time in the sun and liver cancer is often secondary cancer, so there might be no family link. Perhaps remind her that one in three of us get cancer, so unless she's got a symptom, she just needs to know what to watch out for and hopefully her GP can help with this.

Unless there is a definite reason for doing so there is no way that she would be sent regularly to be seen by a skin specialist "just in case". If she wanted to allay her fears after all options had been covered she would have to pay privately.

I really think that as she has asked for this information, then you should show her what you have found, but also say she should talk to her GP about it to get a proper ( or more balanced) overview.