Doctors try to avoid such situations, and, I think, for good reason. They add extra stress to what is already a difficult task -- stress for the surgeon and also stress for the patient. Such extra stress, in turn, is likely to diminish the prospects for success. And a surgeon who performs a major operation on his mother may then be acting unethically by not giving his patient the best chances of a successful outcome. If so, he should step aside, even if his mother would prefer to be operated by him.
But what if some surgeon is, and is known by his mother to be, especially cool and unemotional, thus adding no stress to the proceedings? Or what if he is much better than the other available surgeons so that his greater skill more than makes up for the extra stress? In such cases, I would think, there's nothing unethical about him doing the operation himself -- provided, of course, his mother agrees. So, as I see it, performing a surgical operation on a loved one is not unethical as such, it is unethical -- often -- only because of its likely impact on the chances for success.
To see whether this makes sense, you can consider a more homely example. Imagine your spouse has a life-threatening accident and needs to be driven to the next emergency room at once. It's probably a bad idea for you to do the driving -- you would probably not be at your best. If so, you ought not to endanger your spouse's life, and your own, by insisting on being the driver -- provided there's another good driver available who is much less affected by the emergency. But if the only other available driver is your neighbor's son, one month into his L-sticker, then you may be the better bet and your doing the driving is then beyond ethical reproach.