Sister Complex (3.5e Flaw)
From D&D Wiki
You have feelings for a sibling.
Prerequisite: Character must have a sibling.
Effect: When you are near or around your sister/brother you do your best to impress them one way or another. Whether by playing it cool or goofing around, is your choice. When they are near, you try to constantly be at their side and engage them in conversation or activities. When teams are built, you want your sibling on your team. When the group has to split up, you insist on going with your sibling. When she/he isn't around though, you seem to be distracted thinking about her/him and suffer a -2 penalty on your initiative and concentration rolls.