Fleur could tell her sister didn't fully believe her. So yes, she did have a bit of a guilty conscience. Gabrielle would in no way approve of the truth if she knew it and she didn't want to get into a fight with her this early in seeing each other. "You would be tense too, eef you 'ad to teach a bunch of teenagers all day."
Making a second glass of wine, Fleur moved to the couch with a glass in each hand, handing one to Gabrielle when she passed the chair to sit on the couch and take a sip.
Chuckling, Fleur could only imagine what her sister would do to show the boy. "'e ees very dull, eesn't 'e?" she took another sip, "She seems like a girl who needs more...divertissement." ...entertainment.
Fleur scoffed at the idea of parental permission. She knew her parents would have preferred if FLeur had sought their permission before she took lovers when she was younger, but she had always hated the idea. You should decide who to be with, not society or family.
Taking a long gulp of her wine as her younger sister guessed, she knew she might need the alcohol to ease her nerves if Gabrielle was going to quiz her like this. "Zhere ees a lover." she stated calmly and gave no more information.
Fleur looked away slightly nervous and quickly changed the subject. "So, Gabrielle, do you plan to stay wizh me een my chambers, or would you razher I let you stay wizh your friends een zhe Slytherin dorms?" She knew if she asked, Dumbledore would set up another bed in the girl's dorm. And that would mean Lavender wouldn't have to stop the nightime visits.