Need Advise and Prayers

I was hired as a temp in the accounting department to cover this ladies position while she went on maternity leave. Her sister works here and we worked together for about 12 weeks. The lady came back off of maternity leave and the company hired me on full time and cut the sisters hours down to 2 days a week. So now all three of us are working together and both of the sisters are making it very stressful for me. The tension in this office is so stressful. Everyone liked me while I was a temp and now that I am full time and the girl that has been here a year her hours are cut...I feel horrible and my boss says hang in there everything will be ok.. I don't know what to do?? Any sugguestions are welcome.
Thank you