Hi
Not sure my answers will be what you are looking for, but here goes. I believe that James was Martha's stepfather. She was born in c.1836, and James and Sophia married in 1843. Sophia was 16 when Martha was born and is on the 1841 census living with 2 other young women, and Martha.
I was also on Brownsea this summer for our first Grandson's baptism. If you have family connections with former residents, the Parish will permit you to marry there, and our eldest daughter was married in St Mary's in 2009. This time, without assistance, I was not able to identify James' gravestone - the lady who is usually in the church in the summer has a map of marked graves which a previous rector researched and James' had identifiable initials on the stone when the map was produced. There are either 2 or 3 of Martha's children buried there in unmarked graves.
I did manage to trace Edward. When he retired (and finished travelling round the world) he moved in with his younger sister Clara in Florida (neither had ever married). She had been a schoolteacher, and they died, and are buried in Florida - Clara in 1942 and Edward in 1945. A kind person sent me detail from the Cemetery. Martha and William had one (or possibly two) further children, born in the US and I had some success in tracing the family of Thomas who was born in 1872. I have not been able to find any more detail about Martha and William - they emigrated to Columbus, Ohio, and Thomas was born there. I was unable to find a death record for either of them, and have not since picked up that line. I also have no idea what happened to Sophia after James' death.
Happy to share further if you want - send me a PM.
Richard