Lots of grandparents have a bond with grandchildren they hadn't seen for a while, and they accomplished it without daily babysitting.
Figure out what you can manage FOR YOU. You work, you have responsibilities around the house, your husband travels for work. Each of these families has 2 working parents, but so does your family (you and your husband).
If both kids (daughter, stepson) feel that watching 3 kids under 4 is easy, great. Each of them takes 1 day with the other's kid(s). If that's too much, then they have an inkling of what this is like for you. Sit them down and explain the situation. You are working, and you are home alone (without Grandpa) on the off days with 3 children. You're doing sleepovers so your grandson's parents can have alone time. But you have no alone time, either with your husband or just with yourself.
Everyone's working except the stepson who is still looking. They can afford - they must find a way to afford - some day care and babysitting. You helped out in the short run, which turned out to be a longer run than you thought.
You and your husband have 2 children between you and 3 grandchildren. You want to give more time to the 2 new ones in town, and that may mean you have to (and it's time to) give a little less to the "sweet boy" you're so used to. So I suggest you cut way back, and explain you are exhausted and unable to care for 3 kids. You can offer your daughter's son 1 sleepover a month, and you can offer the 2 other kids a sleepover once a month, but not on the same night. That gives each married couple a big date night a month, and leaves you with 2 weekends for yourself and your husband without grandchildren.
If you want to give your stepson one day a week while he's job hunting, great. He can arrange his interviews (hopefully) and certainly his intense, concentrated on-line searching, for the times when you are available. But he has to have back-up day care for when interviews are scheduled at the convenience of the company.
If you have the means, perhaps you and your husband can assist financially with some babysitting for him. During the summer, your SS and DIL may be able to find a college student who wants a nanny job (a lot of early childhood students like to get experience in this way), and perhaps you and your husband can kick in a little money to help. That allows you to be supportive without doing the work yourself.
And your daughter's son can start weaning off constant care from you - you gave your daughter and SIL some phenomenal help when it was necessary, but you're getting tired. Surely they can be supportive of YOU, and surely they can all see the benefits of their son having 2 little cousins in the area. This is a benefit for them, not just a negative because you get to be a grandma to them as well.
I think you need an honest conversation with your husband to figure out what you can manage and offer, and then a sit down with the kids to explain. Set a deadline of 30 days for them to make other arrangements from the schedule you are currently doing.