I don't allow daycare kids to touch the baby regardless of relation. My own ds isn't allowed to touch his baby sister during daycare, as it is tougher to supervise safe interaction and also the other children will think it is okay.
For safety reasons, baby is always only for the adults to touch during daycare.
It might be easier to enforce this rule when dcg and dcb are both in your care. I tell parents that I have this rule and why if their child brings it up.
I never had an issue with sibling sets following the "no touch baby" rule during daycare even though their parents would let them wrestle/hug/change diapers at home.