Terrified. my sister kissed my 9 week old on the cheek and i noticed she had a cold sore after. what will happen if my baby gets it?! is it unlikely?

Unlikely: A kiss on the cheek is unlikely to spread herpes simplex, though I would discourage any more contact. Your sister shouldn't be kissing anyone with a cold sore- especially young kids. I would not worry. If your child does develop a rash or any fever see your pediatrician right away.

Yes, unlikely: Many "cold sores" on the mouth's mucous membranes are not due to viruses. If your sister had an outbreak of vesicles on the skin adjacent to the mouth, they are likely a benign herpes simplex virus, for which anti-herpesvirus medication is very effective. This common virus is not the same as genital herpes. Your infant is old enough to have a functioning immune system, and infection is unlikely

Unlikely a problem: Your 9-week old infant likely has some protective antibody against Herpes I (responsible for cold sores) from your blood. It is also not likely that a kiss on the cheek would inflict an infection. Eventually most, if not all, people will be infected with HSVI even though most will not have cold sores.