guarantor for rent query

[meganwest]

My partner has just told me that she has signed as a guarantor for her sister on a rented property. My partners parents (unemployed) basically blackmailed her into signing this form at her place of work, and because we have recently moved in together and about to get married, she foolishly put her old address down in an attempt to keep it away from me. We have no copies of this form and her parents are refusing to give her a copy saying that we do not need it as they will cover the guarentee should anything happen. I do not wish to fall out with my prospective in-laws, however I am not prepared to sit on a timebomb, my partners sister seemingly has a bit of a history. I would like to avoid major fallouts at this stage, but feel very uncomfortable with the situation I now find myself in. Can anyone offer a solution please?