As someone interested in glass (as a longtime though casual collector, not as an artist), I can comment on the Murano/Burano option. A lot of the best glass shops in Venice (as well as some very low-end places) are on Murano. However, the part of the island you'll probably see if you head over there due to an interest in glass is not particularly atmospheric or attractive. On such a short visit, I would not rank visiting Murano high. If you spend some time walking around Venice, you will probably pass a few glass shops--some better than others (and some hair-curlingly awful, full of schlocky imports). There are some pricey places near Piazza San Marco. A bit of Googling may reveal some addresses.
Burano is a totally different thing. It is beautiful--full of colorfully painted houses. Unfortunately, it is much farther from Venice proper than Murano is. Getting to Burano on your own from the Palazzo Cornoldi drop-off point would be time-consuming enough that I'm not sure you'd have time to do anything else except perhaps walk over to Piazza San Marco, which is not far from Palazzo Cornoldi. I'm not a cruiser, but I'd be nervous about setting out on my own to Burano if I had to be back at the pick-up point within 4 hours. There's a lot of water to cross between those two points. If you got turned around on Burano and then had some sort of vaporetto problem, you could conceivably be late getting back to Palazzo Cornoldi. At the very least, I'd have my eyes on my watch the entire time. Cost of a round- trip out to Burano is 19 euros by vaporetto. Murano + Burano (which 4 hours does not allow for) would be 28.50 euros.
Since you mentioned your sister has mobility issues, I want to point out that when you need to cross one of the many little canals in Venice, you usually have to go up at least 8 steps before crossing the bridge, and then down 8 steps. Go to Google Maps, find Venice, and then zoom in to observe the many little canals and bridges. Even a short walk can involve several bridges, and you sometimes must detour off what you'd think is the shortest route even to find a bridge that will get you over the canal you're looking at. As it happens, there are ramps for the bridges (I think all of them) between Palazzo Cornoldi and Piazza San Marco, but that's an unusual situation that is not likely to hold if you range far afield.
I'm an art person rather than a history person; I haven't been to the Doge's Palace. I know it's a place many people like to go. Neighboring St. Mark's is also popular; the church interior is spectacular. You'd need to pre-book entry to the church so you don't risk spending a lot of time in a ticket line. I'd do the same for the Doge's Palace if it is possible to do so. I know the Secret Itineraries Tour at the Doge's Palace can be pre-booked and sometimes sells out.
If I had 4 hours in Venice I'd probably prioritize a vaporetto ride down the Grand Canal. A one-way ticket is 9.50 euros. I assume you need to exit the boat and re-enter for the return trip, for a total cost of 19 euros. I don't think it would be a good idea to plan to cover the distance San Marco to the train station on foot in one direction; it might be too much for your sister with the mandatory return time at Palazzo Cornoldi. Expect the vaporetti to be crowded. You might have to stand, though if your sister is visibly a bit infirm or elderly, someone will probably offer her a seat.
Alternatively, you'd have time for a gondola ride. You'd want to plan ahead of time which direction to walk to find gondolas on one of the small side canals. It's much better to take a gondola through the small, quiet canals than to dodge large boats (and their wakes) in the Grand Canal. There are certainly pick-up points not too far from Palazzo Cornoldi, but I don't know exactly where. The daytime cost of a 30-minute gondola ride is 90 euros for the entire boat.
