Could my baby(6 months old)get infected of herpes encephalitis from her cousin( 4 months old) only to with her?

Transmitted at birth: It is most likely the 4m cousin acquired the germ from her mother during direct contact during labor or delivery. The germ transfers to baby & sets up in the system, gaining access to the blood & then moving to the brain. It is unlikely that your child could get the illness in any form during a simple visit to the other child's home. It is not passed in the air.