Why does Linda tells the boys "Get out of here both of you and don't come back"

Although she doesn't know it absolutely, it is already too late to save Willy, but she tries:

LINDA: Get out of here, both of you, and don’t come back! I don’t want you tormenting him any more. Go on now, get your things together!

Oblivious to Willy's deep suffering, Biff and Happy earlier abandoned their father at Frank's Chop house where they were all having dinner together:

LINDA: You’re a pair of animals! Not one, not another living soul would have had the cruelty to walk out on the man in a restaurant!

BIFF (not looking at her): Is that what he said?

LINDA: He didn’t have to say anything. He was so humiliated he nearly limped when he came in.

HAPPY: But, Mom, he had a great time with us...

BIFF (cutting him off violently): Shut up!

(Without another word, Happy goes upstairs.)

LINDA: You! You didn’t even go in to see if he was all right!

BIFF (still on the floor in front of Linda, the flowers in his hand; with self-loathing): No. Didn’t. Didn’t do a damned thing. How do you like that, heh? Left him babbling in a toilet.

LINDA: You louse. You...

BIFF: Now you hit it on the nose! (He gets up, throws the flowers in the wastebasket.) The scum of the earth, and you’re looking at him!

LINDA: Get out of here.

What Linda doesn't know is that without his sons, especially Biff, Willy has nothing. And with his sons, especially Biff, Willy has to face the lies that make up his life. So Willy just can't go on. There is no solution; she can't kick Biff out, and there's no way he can stay. Her husband is hopelessly doomed.