My Dad and Mum have recently split up and are still fighting, leaving my sister and I in the middle. What should I do?

Unfortunately kids, this is what adults and parents do at times.
Depending on your ages perhaps ignore mum & dad having a squabble and go and read a good book. If you can talk about it with either of them sometime after the arguing and tell them how you feel, this would be good. I'm sure they will understand and reassure you both everything is ok.