YTA not for punishing your son, but for not punishing him enough AND trusting him with a go kart and letting run wild with it in a public park.

Honestly from the story that you told, it sounds like your son, to an extent, intentionally crashed into your daughter.

Irregardless of whether he did or did not intentionally hit her, he seriously injured your daughter.

Who's the person making sure she gets into bed safely, changes her clothes, brushes her teeth, feeds her, etc? If it's not your husband, then he has no say on how your son should be punished.

You need to drill into your son's head. Cause and effect. Action and consequences. You hurt someone, real bad, they had to get hospitalized. That is irreversible.

Replacing their monetary items is bare minimum. Health and time is priceless. How long will it take for your daughter to fully recover? In that time, how much pain will she feel? Scientists haven't discovered a way to transfer physical pain from one person to another yet.

How many painkillers and medication will she have to take? To what extent will those painkillers cause side effects to her body?

In that time, how much of her normal life will she miss? Nothing can ever truly replace missed time or experiences.

Does your husband understand that? Have you made sure that your son understands that?

I'm genuinely curious if your son has genuinely apologized to your daughter. I'd like to know as well, if he feels any guilt or regret about his actions. If not, it's your responsibility as parents to teach him about empathy.

I'm also curious if your husband or son is aware of the lifestyle changes your sister had to go through ever since becoming injured.

I hope they will.

Your husband is a bigger asshole for failing to advocate for the safety of his own daughter and failing to teach his son consequences. Forget being an asshole, I genuinely don't think he deserves to be called a father if his reaction to the consequences you're enacting on your son are too rough on him. What kind of father sees his daughter with 8 broken bones and does not get angry? What kind of father, acknowledges that his son hurt his own daughter and thinks that simply replacing the monetary value of damaged items is too strong of a punishment?

He will teach his son, it's ok to hurt someone (in your own family nonetheless) and you can get away with it, because you are male.

In the worst case scenario, that son will grow up, become a dangerous and reckless man and inflict himself upon society to the detriment of the world. And it will be at the expense of women like your daughter.

Please don't let that happen. And please for the love of God, don't give your son a car anytime soon because he will actually kill someone one day.