If the dog has never been alone, how does anyone know that it will have separation anxiety? Just because he barks, or runs around like a fruit cake when your mom/sis leave, doesn't mean he won't calm down after a few minutes and return to his chair. My opinion would be start doing trial separations. Leave him alone, drive around the block / walk around the block, come back in 10 minutes and see how he is doing. If it hasn't escalated past yipping, next time make it 20min.

My big boy has separation anxiety to a small degree. He always has to be with us when we are home, or he will start howling. When we leave the house, he will howl for a good 5 minutes, maybe 10 max. Then he goes about his business and sleeps. (Sometimes even taking advantage of our absence and sleeping on the love seat )