It does not look like lens flare to me - which is caused by internal reflection and scattering between lens elements. Normally induced by light falling into the lens at oblique angles. Avoiding direct sunlight falling onto / into the lens therefore helps, e.g. with a lens hood. Better multi-coating as well as fewer lens elements (non-zoom) would also help, but the Sigma would probably be quite good anyway.
Although some direct sunlight does fall on the lens the direction is close to the lens axis. The green fringes looks more like chromatic aberration to me - made visible by the extreme contrast. A bit like a step response showing up electrical system behaviour...
The following could help with your existing lens:
A smaller aperture like Chip suggested.
Keep the very high contrast part exactly in the centre (on the lens axis) if possible, and crop later.
With other lens options:
As above
If you used a zoom try a fixed focal length as they usually have better colour correction.
Try a step up in colour correction (apochromatic)
