Is it normal to kiss relatives on the lips?
Depends.It depends
Yes, some families kiss relatives on the lips as a normal expression of affection, while others find it uncomfortable or too intimate. What matters is that everyone involved is comfortable with it.
Family affection varies a lot. In some households, a quick kiss on the lips is an ordinary greeting or goodbye; in others, kisses are for cheeks, hugs, or not much physical affection at all. Neither custom is automatically strange.
The important part is comfort and consent, not whether someone is a relative. You can set a boundary or offer a hug instead, and kids should be allowed to say no to kisses without being pressured.