This feature is on Durdham Down just off the Westbury Road not far from the Water Tower. It is possible for this to be an old Barrow, but it could also be the result of tree roots pushing the ground up. An old map-1889- shows Druidical Stones about three quarters of a mile away near Druid Stoke. Down load the pics and zoom in and see what you think.
Equally boringly, the mound is marked on the Ordnance Survey 1st edition (1881) as a tree-clump circle, while O'Neil and Grinsell (in trans Bristol & Glos ArchSoc 1960, page 100) note that the name 'Seven Sisters' derives from the seven fir trees originally planted there.
Whilst I was at Clifton Suspension Bridge in late Dec I saw an interesting topic on the information board for the Downs. Info on the 'Seven Sisters' said "A circle of pine trees, five of which are original, having been planted about 1871. The two added in 1991 replaced losses from the 1990 storms. They stand on a slight mound situated on a high point on Durdham Downs, a location thought to have been a Bronze Age round barrow dating from some 3-4,000 years ago"
Nothing shown on OS map. Nothing mentioned on the 'Magic' web site.