something like this should be ok..
Dear Sir / Madam,
This is to inform you that I would like to invite my son xxxxx and his wife xxxxx to live with me (and my husband???), after she receives approval from the Home Office for a spouse visa and arrives in the UK, they will be staying with me, until they are settled and find suitable accommodation.
I own/rent a x-bedroom house at the above address, which is currently occupied by my husband and myself. I have no problems with overcrowding in accommodating my son and his wife in my house. I have attached my council tax bill/tenancy agreement as a proof of residence/ copy of deeds/photos/land reg docs etc..
If you need any further details with regard to accommodation, please do not hesitate to call me or email me at xxxxxxxxxxxxxxxxxxxxx
Yours Truly
