Hello, I am a solicitor with 20 years experience. I will try to help you with this.
That is an interesting question. So she will be working for you and in order to fulfill her work she will be staying in a caravan on your land. I take it you will not be charging her rent.
She will not get any sort of tenancy rights unless she has exclusive possession of premises at a term for a rent. She will not acquire any sort of squatter's rights if you make it clear that you give her permission to park her caravan on your land for the time that she is working for you up to a maximum of 6 months. You need to make it clear that the permission to park and use her caravan is dependent on her working as your housekeeper and she will have to leave the moment that employment/contract ends. I think it would be sensible to get this agreement in writing and you may need to contact a solicitor to get a reliable agreement drawn up.