"She seems so sure of her position that it made me question mine."
What IS your position?
"I'm neither. I'm the manager who will have to resolve it her line manager can't" you're the managers manager? And you're on mn asking? I find it quite worrying that this suggests you don't know how to perform that part of your job! Have you received no education/training on personnel management? Or do you lack the confidence to support their line manager?
It sounds like the line manager has already discussed with A? And A is continuing in her poor behaviour?
Personally, as the line manager I'd have pulled A in for a chat as soon as the issue was apparent and been VERY clear that such behaviour was unprofessional, unacceptable, illegal and wouldn't be tolerated.
That if they didn't improve their behaviour immediately (availing themselves of the counselling, even taking a LITTLE time off sick if necessary) I'd have no choice but to take the matter higher, and that if I had to do that they may well be facing disciplinary proceedings that could include gross misconduct with a risk to their continued employment.
I'd then if possible (business wise) have sent them home for the rest of day to cool off BUT being very clear that when they came back to work I'd expect much improved attitude and behaviour.
I don't think an apology can be forced but I'd think badly of the employee if they didn't, at the latest upon returning to work, apologise both to B and myself as line manager, because it never should have reached that point.
I'd then speak to B assuring her of my support and that I was dealing with it.
I'd also speak to C to check how they're coping and to ascertain how it was affecting them, supporting them if necessary too.
I'd be doing a weekly follow up with both A&B for a few weeks to make sure there's no covert discrimination/bullying going on and I'd be watching A closely. Any indication its ongoing or not improving I'd be passing the situation up higher.
I've not struggled with conception but I have had losses and gynae issues and at one point was seriously concerned motherhood would never happen for me. At that time a colleague became pregnant via Ivf and she kindly told me before others. It was very difficult to hear, but I congratulated her, went home, sobbed my heart out (and not just that night) then went to work and got on with my job without being a cow to my pregnant colleague! I wouldn't even have dreamed of acting like that!