So, in my opinion, you must discuss the risks and then first go ahead with aneurysm surgery
and then with the knee surgery
. Depending on the location, risk of hemorrhage, risk of rupture, family history, etc, your surgeon will estimate the risk.
Also ask your ortho specialist whether there are other options for you except knee replacement surgery. Then if your neuro-surgeon feels you can wait for a year or more and have MRI in between to monitor progress, then you can wait this time.