I'm going to react with anger when I hear my sister in law went through her car's windshield
Thats an odd thing to expect. Why in particular are you expecting your sister in law to go through her windshield?
Well I read it on reddit that it happens to people who don't wear seatbelt and speed.
Even though they are family, it is okay to not agree with them about their actions. It can be upsetting to see the doing dangerous stuff.
She won't die from it, also it's her own fault for not wearing her sfm.
Going through a windshield often means death or at least severe injuries. Not only is she putting herself in danger but also risking the lives of others. You might want to talk to someone close to her about her dangerous driving habits before something bad happens.