My son is 3years old and when his paternal grandmother babysits him she takes a bath with him naked.?

Is this behavior okay and I am just over protective of my son and think its disgusting? I am looking to speak with her about it mind you she is a stuck up 65year old women who has never had to work a day in her life and barely raised her kids on her own due to a nanny.how do I approach her with this without being the black and white, blunt, foul mouth person I am?Only found out because my son is starting to form sentences?Thought it was odd he took an interest in my although I quit feeding him when he was One. The only person that gets naked in front of him is his 8year old brother when they take showers together and even that is seldom. Help. Thanks.