Linux

I personally think you should just allow cookies indefinitely. There are honestly so many bigger risks from phishing and other forms of social engineering that as long as your family isn't leaving their computer unlocked in a public place, I wouldn't say there's really too much of a risk in leaving cookies enabled.

I apologize that this doesn't exactly answer your question, but I'd like to suggest an alternative. I'd like to also ask, is your family using a password manager by any chance? And if so, are they making use of passkeys on supported websites. Many modern websites, including Google and Facebook, support them. And they require virtually no interaction aside from unlocking the password manager. It's still a form of two-factor authentication, but it's far more convenient than anything out there.

I also don't really think you should try to force Linux on people who aren't particularly comfortable or familiar using it.

I worry they'll get frustrated to the point that they'll go out and splurge on new macbook air when they already have a perfectly functional laptop with functional OS.

If you're worried that they're going to go and do that, then Linux might not serve their needs. Linux might be a fully functional desktop system, but it's also one that isn't an out of the box experience either. There's certainly been a lot of improvements, But I don't think that any Linux Desktop Environment is ever going to reach the same level of intuitiveness as something like Windows or macOS. I would certainly love to see it that way. But I think it's just an issue of the people who actually use it.

I understand looking out for family and ensuring they don't spend excess amounts of money. But you also shouldn't take it upon yourself to try and dictate how your family uses the computer either. Linux wasn't built as a commercially supported desktop operating system with years of full-time researchers studying topics surrounding human computer interaction with a multi-million dollar budget. It was built to be a free as in freedom alternative to the mainstream systems that are available (I personally call it "The problem solving platform" for this reason) by a loose knit group of volunteers who love computers and know a lot about them. Most people who use a computer use them to do work, and not really for promoting a personal agenda.

I'm not saying that you shouldn't try to get your family members to use Linux, I'm just saying that you shouldn't force them. You should put their best interests first that can help them.

I'm sorry to go on such a long rant about this. I just see a lot of people who I believe to vastly overestimate the willingness of others in certain places. And the whole part of "worrying about someone spending their own money" just kind of struck a red flag to me.