I have seen successful granny bungalows built, and it can be wonderful for you all, but the success may well also depend on being sure about legal position. Sorry to sound negative on what sounds like a really loving family situation, but please ask yourself ( I don't need the answers, but you may):
Is she buying the whole property, with you as guests or as tenants? Are you going to be dependant relatives? This may affect her ability to mortgage, or get benefits in the event of her redundancy/ illness.
Are you actually splitting the property into 2, with separate utility bills/ rates etc? This may help with a lower valuation for inheritance purposes (if, in fact, your estate including this would have an tax liability)
If you split the property, how would separate access be sorted if, in the event of your passing, it would need to be sold separately?
Do you have other children to consider? Arrangements you make now must be clarified in your will.
Divorce, bankruptcy ( A friend lost her home because her new husband's business went under and creditors went after the property).
You do need to be clear about ownership of your space - in the event of care home, the first of you will be OK, but if both of you need care, and it is sold separately, it is possible that Social Services may cause her some trouble.
What would happen if life changes and she needs to move away? Would you be willing to go too/could you buy something local/ would the main house be saleable separately?
Other thing to consider are her (potential) parents-in-law's needs