Very.  The short answer is that the FAA certifies each glider for a certain maximum weight and requires that pilots do not exceed it.  Ignoring certified limits on an aircraft is illegal and unsafe, so we don’t do it.  Limits are 245 pounds for ASK-21 passenger gliders
Whether you can sit up front or not is both weight and pilot dependent.  Some of our pilots prefer to fly from the front seat.  In addition airplanes (even really big jet airplanes) are like teeter totters and the distribution of passenger weight in the front and back seat affects the balance point and thus the aircraft handling.  Smaller aircraft like our gliders are more affected by small changes in the distribution of pilot and passenger weight.  Your pilot knows and understands those limits.  Thus, the maximum weight limit per passenger is firm and it will be up to your pilot to determine whether you can sit in front or not.
However, the view from both the front and rear seats is extremely good and the rear seat of the ASK-21 is quite a bit roomier than the front seat.  Some passengers prefer the rear seat as they can see what is going on with the pilot and feel more secure sitting closer to the wing rather than out front.  The only promise we can make is that the rear seat has an excellent view -just like the front seat.
See the video at the bottom of our passenger page to see what it looks like from the rear seat of an ASK-21.