I would also add the enabling issue in this particular family.
Linda enabled Willy's infidelity by not questioning her husband enough, not getting to know exactly what was going on on his trips. After so many years of marriage, you must question exactly how ignorant she was on the situation. She is still enabling Willy, as she still has not stood a firm ground on getting help for him. Instead, she pretends that all will pass, and that those episodes of insanity she has to just put up with. She also wants to continue enabling Willy by asking the boys basically to "work around him". Linda also chose to basically blame Biff for everything instead of opening her eyes to see what was going on. She didn't set the record straight with Biff, nor Happy- it was all a matter of not facing the elephant in the closet.
Happy is also an enabler: He was tapping around the issue if Willy's illness and each time Biff tried to bring his father back to reality, Happy would just giggle-it-up, or say something positive to cool off the situation and pretend all is OK. This was not helping Willy, nor Linda, land on what the real issues were.
Biff, for the fact of not being an enabler, just ran away from that enmesh situation altogether. Because he was not willing to enable Willy's infidelity, he broke ties with everyone- However, he did not face the situation either, and in the end it all ended in Willy's imminent death- all because nobody took the first step to say "enough".
Both sons are affected by Willy's actions. Happy suffers to make his father notice him. Each attempt to win his father's love and repect is a reflection of Willy's own attempts to gain the respect of those around him in the business world. Each time we see Willy meeting with someone, or begging for an extension, we see Happy, either in the present or the past, desperately seeking his father's recognition. His failures to win his father's favor is a direct result of his father's inability to win the favor of those around him in the business world.
Biff's situation is more evident. His entire life has become an escape from his father's infidelity. He has almost deliberately chosen failure as an excuse for never earning his father's praise again. Of course, in choosing this, he too has become like Willy, in that he never "makes it" in any profession. Thus his life has also fallen into the pattern of failure that we see with Willy.
Finally, Willy's infidelity has forever altered Biff's relationship with Linda, and by extension, other women. Biff is perpetually angry at his father for the mistreatment of his mother, and perpetually angry at his mother for allowing herself to be treated in this manner. This anger manifests itself in his relationships with other women as well; hence, Biff is unable to keep a steady girlfriend.