In brief terms, (you are aware of this) there is no relationship between your landlord and your tenant. The only relationship is between you and your tenant. Your landlord is out of the equation.
Notice by one tenant to end the tenancy agreement (provided it is within the terms of the agreement) is notice of all tenants and one tenant can give notice on behalf of both of them. If the other tenant then remains in the property, then the other tenant needs to “negotiate” a deal with the landlord (you).
I don’t know on what basis they kicked your brother-in-law out because they actually have no legal right to do that.
Whoever is responsible for the damage is obviously liable to pay for it and if they are joint tenants, then they are jointly liable. If one did the damage then you can chase both of them and whoever is innocent in all this will have to pursue the one that did it.
You ask what your situation is, you pursue your brother-in-law and his girlfriend jointly for the full amount of the debt in respect of the rent and the full cost of the repairs and cleaning. Let them sort out between them who owes which proportion. If you have to take them to court, then you take them jointly as first defendant and second defendant.
They had a lodger with two dogs and as they have now vacated, there is little you can do about that although added to the cost of the cleaning.
If she has given notice, and the notice has expired, then abandonment doesn’t come into it.
You can normally only claim a couple of weeks lost rent if you need a couple of weeks to put the property into good order.
. Can I clarify anything else for you?
I’m happy to answer any specific points arising from this.
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