@Abby40 - You will get lots of great advice here from experienced hosts, so it is wonderful you thought to ask the community for their advice. As for me, I would not advise you to 'let it go' unless you have your own commercial insurance. The fact is, this woman's daughter is staying in your place and not covered by Airbnb's Host Protection Program. It is unlikely anything will happen - But, if it did, you will not be supported by Airbnb at all, and you will not be able to make any claims for damages or be protected in any way without the Host Protection program coverage (example - daughter falls on your property and injures herself and holds you liable).
I let many things go with guests, but not this one. They need to be registered, and if they are not, I send an altered reservation request and have the guest pay the charges. I will not discount, either - It is a lot of extra stress and trouble for me to have to deal with such a situation and I feel I am more than entitled to my extra guest fee (extra linens, extra wear and tear, etc). This woman likely knew her daughter would be staying there and there's a good chance she hoped to avoid the extra fees. Why should you reward her by foregoing your fees, or even discounting them, when you are the one that was put in a very difficult position by her actions?
By the way, in my 'Additional Rules' under 'House Rules', I clearly state that unregistered guests are not allowed, and I do not accept 3rd party bookings. But it still happens. So know your boundaries, and stick to them. And, ideally, your boundaries will be based on what gives you the most peace of mind, and the most protection insurance-wise, and legally.