J.C.
K.-
Herpes cold sores are very contagious and are very common. Considering your baby is 9 months old and their immune system is pretty established, they should be ok even if they contracted it. If they did contract it, the biggest frustration would be dealing with the breakouts.
Herpes cold sores are a very touchy topic for me. In the early 80s, my newborn brother contracted herpes from a nurse (she had a cold sore and had touched it and not washed her hands before she touched my brother) in the infant nursery at the HOSPITAL. Since his immune system wasn't fully developed (takes 4-6 weeks), he developed a high fever and died when he was 10 days old. I don't mean to alarm you, but Herpes cold sores are nothing to take lightly around a newborn! Again, your baby is much older, so I think they are ok but I would definitely talk to your SIL to confirm if she did have a cold sore.