How did Iris correctly guess that Graham had sex with Amanda Woods?

Question

In The Holiday (2006), Iris had a phone conversation with 2 persons at a time: Graham and Amanda Woods.

Iris: I can't believe that you have had sex with the woman staying in my house.

Amanda: He told you that? Oh, my God! Oh, my God! Oh, my God!

Iris: I thought I was talking to Graham! Could you just hold, please?

So how did Iris correctly guess that Graham had sex with Amanda Woods?

Answer 1

I'd expect it to be a combination of

1. The way Amanda was talking about Graham
2. The way Graham was talking about Amanda
3. Iris knows her brother and what type of person he is.

It's never explicitly stated but I imagine the close relationship held between Iris and her brother would mean she knows what she's talking about.