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Hello my name is Alex and I will help you with this. Please note that I am a working Solicitor and may be on and offline as I have to attend Court and meet with clients, even at weekends. As such you may not get an instant response when you reply, but rest assured I will be giving your question my immediate attention upon return You do not need to wait here as you will get an email when I reply.
Who has signed it please?
Yes it can be. It is the legal doctrine of the unknown principle.
So if you did not know the true identity of the Landlord when signing then it is legal.
If you knew the Landlord was not the sister then the tenancy agreement is not binding.
Can I clarify anything for you about this today please?
If you knew and accepted it - then it is binding.
Does that help?