Will yawning widen my nose?

No, yawning won't cause any permanent widening of the nose. As for making the nose less wide, a surgical rhinoplasty can usually achieve proper narrowing while still preserving good breathing through the nose. We have techniques to narrow the bridge of the nose, the tip of the nose, and even the base of the nose (the alar base), depending on the patient's unique anatomy. Visit with a surgeon who's primary focus is rhinoplasty. He or she should be able to give you some ideas of what can be done and should be able to show you with computer simulations what you can expect.All the best,--DCPPearson Facial Plastic Surgery®

Certainly yawning will make no significant difference to your nose. 
There are a number of surgical options to narrow a nose and some can be done under local anesthesia. Seek out a Board Certified Plastic Surgeon for in person consultation to get an opinion about this.
I hope you find this information helpful.

Yawning shouldn't affect the size of your nose. I recommend having a consultation with a plastic surgeon experienced in rhinoplasty to review your questions and desired outcomes.