Update:
Jessica was not at netball last week, and I had no opportunity to speak to her mum. But on Wednesday (the day before practice) I saw her and asked her to chat with me before practice the next day. Her response to that was to turn to Jessica and say:" I wonder what
she is on about again!"
This made me really angry. It's hard to stay polite when you are angry

but I did. I said that I would like to have a chat with her without the children present, and that she should try to see me before we start.
On Thursday morning the school principal gave me a call, to discuss a telephonic conversation she just had with a "very distressed parent". I was happy to tell her the backstory, and I told her about the mum's disparaging remarks the day before.
I am happy to report that the principal called Jessica's mum to inform her that her child *will* abide by my authority if she chooses to continue playing netball, and that the mum will have to stop undermining me with immediate effect. She also told the mum that she would have to sign the code of conduct, and that she will be bound by the requirements thereof.
Thursday afternoon practice went off without a hitch

Jessica's mum dropped her off and skulked around the corner, but she wasn't confrontational, and she didn't try shenanigans. Jessica gave me a big hug after practice... I am glad that she isn't being "put in the middle" of the grown up's conflict.
Thanks again for all the advice, guys. It really helped me to stay focused and polite.