K.B.
My son was 23 months when his sister arrived and then they were 5 1/2 and 3 1/2 when their younger brother arrived. I breastfed all of them right in front of each other and never had any problems or concerns about it. My kids never asked about it but if they had I would have handled it as I would them asking to drink from a bottle or use a pacifier. I would have simply explained that this is how babies get their milk and that since they are big kids they get their milk from a cup. I don't see any reason whatsoever to be concerned.
Good luck,
K.