UK Property Law
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Your Mother-in-Law is not a Tenant, but a lodger and as such she has no security of tenure.
I take the view that, in those circumstances, you are not in breach of your tenancy agreement as she is not 'a tenant'.
A lodger (also known as a licencee) is one who has exclusive occupation of his or her own room but who shares all other (including bathroom and kitchen) facilities with the owners or tenants of the property.