She was trying to understand, she really was, but he wasn't giving her a whole lot to go on here.
"How do we talk to each other?" said Tonks, her brow furrowing as she struggled to see what Charlie was seeing. "It's part of being in an intimate relationship, I suppose. You know how it is, getting close to someone like that. That sort of intimacy changes how you talk to your partner. But it doesn't change who you and Bill are to each other, or who you and I are to each other. The conversations are just going to be a little different." She squeezed his arm. "When you have girlfriends, is your brother not your brother anymore? Am I not one of your best friends? Your girlfriend doesn't replace anyone in your life, right? And it's the same for us. Me loving Bill doesn't change how I love you, and him loving me doesn't change how he loves you. I promise. Like I said, you're stuck with me for good no matter what."