The wheelchair is an excellent idea, especially for someone who needs oxygen. It can help carry the weight of the oxygen, as well as keep your mother-in-law from getting worn out too quickly. I would suggest you contact the cruise line about what their rules are. They subcontract with local operators for their shore excursions, which may have their own rules about oxygen bottles and wheelchairs, but they still may be able to offer some suggestions. If you booked through a travel agent, ask her/him to help you find out more information too. Have a great time! I have seen lots of cruise passengers in wheelchairs, and they usually seemed to be enjoying themselves.
Another question - are you driving or flying to the cruise? If flying, be sure to check on the airlines' rules regarding the oxygen!