What has she done?

[anothersecond]

NC as definitely do not want to be outed on this.
I've just got out of my step daughter why she has been behaving so differently (grumpy, argumentative, sometimes really happy but often keeping to herself) she's having an affair and it's with her cousins husband.
She's been out a lot the last couple of months. Not too unusual, she's 19 and lives with us still.
Yesterday, she came home in tears and after various attempts to get her to talk, yesterday I managed to get her sat down and find out what was bothering her.
My step daughter is really close to her cousin so this has come as a huge shock. Her husband is 36 and they have 2 young children together.
My step daughter says they love each other- give me strength.
How do I get my step daughter to see the bigger picture here?
She was upset last night because they had an argument.
She is such a lovely girl. How do I get her to value her worth? Lying and cheating never work out well.

As with others I think the blame for this lies at the cousin's H's door. Teenagers can be very passionate about lots of things. He should know better.

I'd start by telling him that you know and that he needs to tell his wife. You can't leave her unaware. He is not the man she thought he was.

I think its important to find out how old she was when anything sexual happened between them. If it was as a minor then you have to report it to the police. If it happened when she turned 18 then talk to the wife and let her know about the affair. That's the right thing to do.