Evelyn Widmer-Schlumpf's given name is "Evelyn" and her (adopted?) family name is "Widmer-Schlumpf", so the correct way to refer to her just by her family name would be "Widmer-Schlumpf". [I am Swiss BTW.]
Please keep in mind that many countries / cultures, even Western ones, have a naming tradition different from "first given name / second given name / single family name".
I was typing on my mobile phone and was getting tired of typing out the full name, but you are correct (incidentally, it wouldn't be different in Germany).
Please keep in mind that many countries / cultures, even Western ones, have a naming tradition different from "first given name / second given name / single family name".