Complex sentence: Advertisements paid for by Mushroom implied that the Wilson sisters had a sexual relationship; the band was furious with this and relations were already strained when the group and their label tried to re-negotiate their contract.
Split into simpler sentences: Advertisements paid for by Mushroom led fans to believe that the Wilson sisters had a sexual relationship. The band was furious with this and relations were already strained when the group and their label tried to re-negotiate their contract.