Next question should be what are they doing about it? A repeat blood test would be handy!
Why did you have the test done, and was it abnormal white blood cells or an abnormal white blood cell count? Two different things, infections etc will alter white cell levels, other conditions will produce abnormal white cells.
Worrying should be turned down until next step known. If it was a nasty, then I’m fairly confident it would not just have been down to MiL to tell you and leave it at that, rather it would have been “please attend…….”