It's not uncommon for patients with knee arthritis to have many other health problems at the same time. After all, advancing age is linked to both osteoarthritis and other diseases and illnesses.
Some patients are afraid if they don't do both knees at once, they won't ever have the second knee done at all. Being laid up all at once rather than having two separate perations makes more sense to them. Your concerns about your aunt's health are well stated. The surgeon will take them into consideration when advising your aunt about the knee surgery.
If it's possible, it may be a good idea to visit with the doctor either by appointment with your aunt or by phone if you are far away. If your aunt has both knees replaced at the same time, she may have a longer hospital stay and more rehab before going home. She may also need more help once she returns home. Let the doctor know if there is family available to assist during the transition.