I would suggest that you find out who actually owns the property. If you don't have any paperwork from your partner or her son-in-law, then download the form available from the Land Registry's web site.(posted 8317 days ago)http://www.landreg.gov.uk/info/faq.asp?id=48
You can then find out who's name the property is in. If its your partner, then you've nothing to worry about. If not, then you can start investigating - write to the solicitor who acted for the son-in- law and get a copy of the transfer and any other paperwork which your partner might have been asked to sign.
As Iwasduped says, its possible she might have signed papers under duress. Normally you would be told to get your own legal advice before signing anything.