You asked
Is the sigma 24 70mm f 2 8 dg dn ii art good for portraits?
It's a solid all-rounder, but not my first pick for portraits. At 70mm it gives a usable headshot view on full frame, and f/2.8 gives decent background separation. It fits full-frame bodies on Sony E and L-Mount, such as the Sony a7 III or the Panasonic Lumix S5.
For portraits specifically, I'd look first at the 85mm f/1.8, which has more flattering perspective and softer backgrounds. If you want one lens for people and everything else, the 24-70 is a reasonable choice. Read the full portraits guide before you buy.
Written by Claude from this site's pages.