Horrible Gramma

Ok, so here's the deal...I got a call from my daughter-in-law Monday. She left me a voice mail (I was at work) asking if I could keep one of the boys when she & my son go to Vegas in August. This is bad. In today's world, it is accepted, Hell, even expected that grandparents will care for their children's offspring on a regular basis, even to the point of raising them. This is the "way it works" so to speak, in most families.
My in-laws rarely keep our boy, and my parents are both gone, so we don't get much "help", and time child-free is almost non-existent for us. On occasion, Stacey or Rosa will keep him over night, and on very rare occasions (usually if I have to work on Saturday, which kinda ruins the benefit) she'll take him to the lake for a weekend in the Summer. But as a rule, if we aren't at work, we are in the company of the (usually) sweet (monster) boy. And he requires a lot of...well, something. I'm not implying in any way, shape or form that I think this is bad, just trying to explain our situation, and why we feel the way we do about this request.
We are not a typical Grandparents (in case you hadn't picked up on that). For one thing, Hunny and I both work full time (I work at least 40 a week, and he usually works 60 a week). My schedule is really screwy, and what time I have off is usually so full, I work harder on my days off than I do when at work...Now I know many of today's grandparents still work full-time, as grandkids are coming along earlier in life. But let's add in to my mix the fact that I have a five year old of my own. I still deal with a little one, usually to the point of exasperation every day. Most folks who are grandparents have children that are grown, or at least mostly grown.
If my (usually) sweet (monster) boy had never put in an appearance, my life would be very different. I would be more than happy to take my grandkids and keep them for a weekend, or a week, or...well, you get the picture. But he did. My life is much better for that, but consequently much fuller, and quite frankly I really have absolutely no desire to chase a 16 month old, Mama's boy who freaks out whenever his parents aren't in sight, or provide the constant attention required by a 4 month old. And deal with my 5 year old. He's a handful all by himself!
I talked to Hunny about this, and my reluctance to do this, and he agreed with me (I love my wonderful, supportive husband!).
She hasn't called back yet, which her message said she would do, but when she does, I'm afraid it's going to start a "family feud." I love my grandkids, but between work, my (usually) sweet (monster) boy, and ongoing health issues, I'm just not up for it. I'm a horrible gramma. And probably a horrible mother-in-law...
Any feedback, comments, opinions or advice would be welcome at this point. I really do love my grandkids, as well as my son & daughter-in-law, but I just can't go there. I think it would be a bad idea, for everyone involved.
Ok, gotta get ready for work...ugh!
See ya laterzzzzzzzzzzz!!!!