Haven't done modelling for Sins, but I do model some stuff for Civ4 mods - and I'm a half-way decent artist with acrylics and good in the use of photoshop. When I took the plunge into modelling, I found that creating meshes is a completely different skill set, that drew more on spatial visualisation than anything else.
Texturing... benefits a bit from painting (because you have some feel for the colours and good blendings or patterns), benefits a lot from being proficient with photoshop/GIMP - but is also sort of a new thing.
Basically, it doesn't draw on the mechanical skills of arts, it's its own thing - having some experience with arts will help to get into it faster (because you can wrap your head around what looks good/bad more easily), but it's by no means a requirement.
So, just go ahead and try, you'll develop the necessary skills as you go along - and every model will be better than the last - just don't give up, if the first one or two aren't supergreat.
Cheers, LT.