Assuming we're talking cis swaps here, there's actually a pretty drastic difference between a swap for just Alec and Viola vs a swap for the whole family! But I'll talk whole family.

Alec...honestly almost makes more sense as a cis woman. The favoritism shown to Bertrand, the religious abuse, the emphasis on the need to control her emotions and present a perfect facade to the world....all of these things would have a more gendered tint. As a result, Alec would be a very femme woman who takes pains to present as humble, self-effacing, compassionate, and charitable while internally still being the rampant egotist and unrepentant sadist we know and love. Also, like canon Alec, she would enjoy crossdressing and would probably be something of a leather daddy. The main difference here is in how she presents herself to the world.

Viola, meanwhile, would lack the unshakeable confidence of his canon counterpart. He would be angrier and more frustrated by his insecurities around his masculinity. His wife is older, more powerful, and physically larger, and babytrapped him when he was eighteen; Viola walks around with a chip on his shoulder feeling like people see him as less of a man than he'd like. Like canon Viola, he copes with both control and the loss thereof—he runs a tight ship professionally and in the household, and is a more involved parent, but also indulges in substance abuse. He finds both shame and pleasure in how Alec dominates him.

Now if we're talking Isabelle also being a boy, the family dynamic is very different. Alec projects her dead twin Bertrand onto him (assuming Bertrand wasn't also swapped) and Viola projects himself onto him, so both parents are very strict and maintain high standards for their son. Viola would be more involved and would prioritize academic success. Alec would be less indulgent than in canon, although no less affectionate, and would prioritize public image and perfect obedience.

Isabelle himself would still be nonbinary, I think? Just AMAB and more male-leaning. He would be better at blending in socially but paradoxically feel more alien, since he can tell he's faking. To try and escape this he would spend a lot of time by himself, and would usually be found in his room composing. Both inflicting and withstanding pain would help him feel relief, and he would embrace his sadism more. He would feel less guilt about Alec's sexual abuse of him and I think would have a deep rooted belief that, if he gets a sibling-child from her, he'll finally feel less alone. Her age doesn't deter him.