There is no doubt that Linda Loman is an exceptional wife to Willy Loman. She is also a wonderful mother to Happy and Biff, that is, whenever she is allowed to raise the boys her way.
The most important points to take into consideration regarding Linda's loyalty to Willy go far beyond her support of his dreams, nor her assiduous attempts to mend the relationship between Willy and Biff. Those are important points, but there is a lot more to dwell into.
First: Linda keeps and protects Willy's role as the man of the house, and as the head of the household- Linda knows every single weakness that Willy Loman has. She knows that he has not been the best husband, nor the best provider. She also knows that he has not done much for her. However, the loyalty that she has for his role as her husband makes her submissiveness look less than a sacrifice and more like a proud role that she gladly accepts to take.
I don't say that he's a great man. Willy Loman never made a lot of money. His name was never in the paper. He's not the finest character that ever lived. But he's a human being, and a terrible thing is happening to him. So attention.must be paid! He's not to be allowed to fall into his grave like an old dog! (Act One)
Second: Linda prefers Willy's happiness and comfort above her own. Linda is a woman who lives, like she says, "from day to day" trying to prevent Willy from committing suicide. She also lives "from day to day" trying to find Willy's soft spot, and trying to get his comforts; for example, her purchasing American cheese instead of Swiss just to give Willy "a change". Moreover, she is willing to hide her anxiety, sadness, and fear by, instead, singing to Willy in bed whenever he is the one who feels sad, scared, or anxious. We know that Linda is holding in a lot, because her humming is described quite pathetically. This happens in Act One when, in the middle of the night, Biff goes to the kitchen and finds Willy's rubber tubing behind the heater. This is another way that Willy plans to kill himself. Meanwhile, Linda keeps humming to Willy, despite knowing this too.
(Biff) is horrified and turns his head toward Willy's room, still dimly lit, from which the strains of Linda's desperate but monotonous humming rise.
Third: Linda's loyalty reflects in her quest to boost Willy's ego.
Linda is fully aware also that Willy is quite preoccupied with physical attraction because, in his mind, that is the key to success. Rather than blaming his lack of selling skills on himself, Willy blames it on his looks. He also blames everyone's strengths or weaknesses on how good, or bad, they look: on whether they are well-liked enough. Linda, as a typical loyal and submissive wife, reassures Willy of his looks, of his selling skills, of his parenting skills, and of almost every aspect of his life. By boosting his ego, she feels, is part of her wifely duties.
Willy: I'm fat. I'm very foolish to look at, Linda. [...] as i was going to see the buyer I heard something about-walrus. And I- I cracked him right across the face. [..] they do laugh at me. I know that.
Linda: Willy, darling, you're the handsomest man in the world. [..] to me, you are.
Hence, Linda takes the job of wife and mother to the extremes, sacrificing her own happiness and sanity in order to keep the family together. It does not seem like this is a tough sell for her, though. Linda seems to be quite proud and willing to do it.