YANBU.
I can't understand how having a baby has become a valid excuse for rudness. There seems to be an attitude, held by some, that as 'it's my baby' and 'I have a baby' you can do and say what you like and sod everyone's feelings, and even minimal boundaries of consideration.
Yes others should be thoughtful to you, and acknowldging your exhaution and shellshockedness, and it might explain, but not justify, rudness.
If you have a baby you should still attempt to be considerate to others, and if you have a valid reason , meaning more than your own neurosis, for not wanting someone to hold your baby, you should attepmt to find a way of doing that without causing offence and hurt.
Calling your SIl who has travelled specially to see her nephew a stranger, is just crass and hurtful, new baby or not.