Does my body still make antibodies if i'm exposed to a cold but don't get symptoms. hoping to protect nursing 2mth old from brother's cold. ?

Yes: Symptoms from a viral infection vary greatly from one person to another. However you would develop an antibody to that virus if infected. Whether you can prevent the child from developing a cold or not is uncertain but do note that a child in the first 3 months of life possessed protective antibodies to many infections and breast feeding may provide added protection as well.