Is it normal to kiss your sister on the lips?
Depends.It depends
It can be normal in families where lip-kissing is a casual sign of affection, while other families find it uncomfortable or unusual. What matters is that you’re both comfortable with it and free to say no.
Families have different ideas about affection. For some, a quick kiss on the lips between siblings is no big deal; for others, cheek kisses, hugs, or no kisses are the norm. Neither reaction is automatically weird.
The important part is mutual comfort and boundaries. If either of you doesn’t like it, switch to another kind of greeting or affection—no big explanation needed.