Mom is obviously a rex and that means 50% of her babies "by odds" will be rex's
Momma rat might be what is called a loose rex.
Loose rex's come from breeding a double rex X double rex where the outcome is said to be 75% double rex and 25% mixture of standard coats and loose rexs.
The loose rex typically has a rex coat that isn't very curly compared to a standard rex coat and whiskers that are only slightly curled.