Not the same thing at all.
Lots of men are sexually attracted to their sisters, but it's very rare and abnormal for a man to be sexually attracted to his mother. About 10% of men and 15% of women
reported having had sexual experiences involving a sibling. The percentage of people who have felt attraction for a sibling but never acted on it is much higher.
Mom-son incest has always been considered taboo in civilized societies because it makes no sense to betray one's father by sleeping with one's mother. But brother-sister incest was not viewed as inappropriate in some historical societies. There is a risk of birth defects, but a woman in her 40s who gets impregnated by a non-blood relative has the same risk of birth defects as a woman in her 20s who gets pregnant from her brother.
Brother-sister marriage was common in one of the world's oldest and longest-lasting civilizations, so maybe the practice has some benefits.