Fleur didn't know why she was so adamant to defend the family she had married into. This was their fight, not hers. As individuals, Percy and Fleur had nothing to do with each other and therefore no need to bicker, but she felt a loyalty to her husband and his family that could not be shrugged off or ignored. Perhaps it had something to do with her relationship with Molly not being strong enough yet. They had come a long way, but she wanted a better relationship with her Mother-In-Law than the one they had. If Fleur had thought about the words she was about to say next, she might have noticed that Molly might not take kindly to Fleur giving Percy so much grief, because he was still her son, and she did still love him.
"Eef you zink for one moment zat your muzzer does not miss you seemply because she 'as six ozzer children, zen you are not nearly so clever as your reputation may state," Fleur said, trotting alongside Percy. "Togezzer zey may compensate for your shortcomings, but no good muzzer could be 'appy while 'er son is estranged. My muzzer always said 'You are only 'as 'appy as your saddest child.' Zink of zee sorrow you are causing!"
Fleur did not stop there. She felt it was necessary to say out loud the things that Percy already knew, if only to needle him further, "Eet ees not just your muzzer 'oo ees miserable. What about your bruzzer? My 'usband? You did not even come to our wedding."