what if they are just living at home in their early to mid twenties and can afford it but just don't want to work the extra day to get it?

No , not if they could do it themselves!!

No, you shouldn't and neither should their biological parent. By doing so you are just teaching them to be lazy freeloaders, especially if it is as you describe and they have nothing else going on.
In certain circumstances I would say it is ok for you to pay like if they are going to school full time and are really hard workers but have too much of a load at school to take an extra day at work.

If they are unable to afford it themselves until they are 25, you might want to do it. You are not required by any laws to do this, but think of your marriage if they are injured and can't pay.

they should learn to take care of themselves in their mid twenties

I've never heard of an insurance company that would allow a step parent to insure their step child. And even if they did, normally insurance cuts off children at 21, they also require that they be a full time student. So I'm not sure how she could insure her step children.

But, unless it would not cost her anymore money to do so, it's really up to mom and dad to do it, not step parent. I wouldn't do it.

No. The idea of being a step parent is to have all the advantages of being a parent, whilst the real parent pays for it.