I have no doubt Smegs planted the seed of doubt in Harry's ear long before William uttered the fatal words. I suspect she had thought through the likely opponents. She could easily charm Charles - flatter him, flutter her eyelids, flirt a little. But William would be a harder proposition, as he was so protective of Harry and he had Kate backing him up - who had probably already displayed some reticence towards Nutmeg. Hence, Smeagol got into Harry's ear....
"What if they don't think I'm good enough Harry
- Harry, I'm worried - I think William will try and stop us, or slow us down so he can put more obstacles in our path"......and more pillow talk about how they must not listen to his naysayer brother or they would never be together forever and ever (insert saintly Smegxit pose here)....So when Wills did indeed utter the words to Harry it immediately cemented Meggsy, his one true love as the source of all truth.