If you leave it too long you will have her established living there rent free, with all the problems that will bring. Decide what you want to happen - it IS your house! - and make arrangements accordingly.
You want all your children to inherit equally when you die? Then someone with no rent-book and no terms of rental will cause problems for the other two, so you need to settle on a fair rent and have a rent-book which lays down how much rent and how much notice is needed for her to leave (either by her choice or yours) and is signed by both of you whenever rent is paid. It should also state what happens if rent is not paid regularly, just in case your "not getting on" escalates into trouble! I agree that you should see a lawyer and get the agreement drawn up properly.
If she doesn't like having it on an official basis within the family, then she might be better renting from someone else entirely.