I find some of the responses on this post surprising.
If the OP's DD was in her own place with a flatmate, she'd have no right to insist that they didn't have visitors over in the afternoon. Early in the morning or very late at night, fair enough, but the afternoon is a perfectly reasonable time to visit and it's only twice a week.
The DD isn't being asked to sit and socialise - OP says the problem is that DD finds the conversation too noisy. Rather than stopping her mother having visitors in the afternoon in her own home, perhaps she needs to try ear plugs, noise cancelling headphones, or even playing music through air pods or similar to drown out the sound of people chatting.
If the OP's DD is so sensitive that she can't manage the sound of three people chatting on the other side of a wall then she needs to figure out how to get her own place - and just hope that she doesn't end up with noisy neighbours!
Having a health condition that means you need to rest doesn't mean you get to impose strict rules on the rest of the house during the daytime.
And bear in mind I am saying all this as an autistic adult who gets very overwhelmed with people and noise. Despite this, I can still recognise when I am being unreasonable and when I need to find my own coping strategies rather than expecting others to conform to my wishes!