No, you cannot get a sinus infection from someone else. Sinus infections are considered secondary infections or complications that usually started out as something else (virus, flu, allergy, etc.) but developed into the sinus infection because the individual is predisposed to catching sinus infections for whatever reason. (low immune tolerance, allergies that weren't properly treated and got out of control, underlying medical condition such as cystic fibrosis, etc.) Therefore, your child can NOT catch the sinus infection from his grandmother. However, here's the tricky part, IF your mother in law's sinus infection is a complication from a flu or cold virus, your ds CAN catch the cold virus or flu from your MIL. If your mother in law's sinus infection was caused by allergies, that wouldn't be contagious. From the sounds of it, though, it's quite likely that your MIL's sinus infection started out as a cold which your ds now has. Whether or not it develops into a sinus infection depends on whether your ds is predisposed to them. For example, my ds has chronic sinusitis. When he catches nearly any cold from anyone, it always ends up as an acute sinus flare-up. My dd, on the other hand, has good sinuses but bad lungs. For her, colds are much more likely to develop into pneumonia than sinus infections but very often end up being nothing but a bad cold.
I hope I didn't confuse you more. My simple answer would be that unless you know for certain your MIL's sinus infections were caused by her allergies, I'd assume she had a cold and that your ds now has a cold. Watch him closely and follow whatever protocal your clinic suggests. I hope ds feels better soon.