Sucking - you mean motive with Grace? been through some options earlier on the thread, but it comes down to (if it's Sylvie) her interference in business which annoyed Sylvie and also that S thought her partner is attracted to Grace (she asked him in the last episode and he did say he liked G as a daughter, but we think on the thread that they were in cahoots about something, but not having an affair, some posters think he may be a transvestite secretly or that he supplied G with drugs - so he was visting her house and Sylvie may have seen his car there and suspected all sorts, or just suspected anyway). If it's Henry, than he was competitive regarding business, feeling like she was favoured over him - but I think less likely.
It could be step-father, maybe he did try it on and she threatened to tell Sylvie, or threatened to reveal some other secret of his - and he had to knock her off. It could also be an accident - either Sylvie or her partner or even Jason got into a fight with her and knocked her out without intention to kill but didn't have the guts to take the blame.Could also be a motive that we don't know about yet.