"The engagement Nicki tells his mum that he..." joke

The engagement
Nicki tells his mum that he has got engaged at last. His mother is happy but a little bit worried as well. She just has to ask him, "Is she Jewish?"
"Of course she is, mum. I`ll bring her to dinner this evening so you can meet her."
That night Nicki arrives with three beautiful women - a blonde, a brunette and a redhead.
"Mother I want you to guess who is my fiancé." says Nicki smiling.
But his mother is not pleased at all. All she wanted to do was to speak to her son’s fiancé one to one first without playing silly games. She doesn`t know where to start. She waits patiently and gives it some thought. When the meal is over, she calls Nicki into the kitchen.
"I know which one she is." She says.
"Which one, then, mum?" asks Nicki.
"The blonde"
"Yes, you`re right. How on earth did you guess?"
"I knew as soon as I saw her, I couldn`t stand the sight of her!"