How do I tell my sisters-in-law & brothers-in-law that they need to help me out more with their mother's care?

It is probably unavoidable that they hear, "you haven't been pulling your weight," so let that go. In your heart, if you are clear that you are not blaming but asking for help, you can ask clearly and gracefully. In their defense, it may look to the, exactly as you described: that it's covered, that you are glad to do it, and it's off their list. If they don't see a need, from their perspective, they don't offer to pitch in because that is YOUR territory.
They need to hear that you need help now. That you'd like to look for a job. That you need to get help with their mother. Then offer concrete suggestions for them to pick up...don't leave it general. "she needs an outting every week...can you take her shopping every other week?" etc. Be patient while they learn how to step in and help. Until they start doing it, they have no idea what you've been doing. Even if you think they should.

JaneB, Thank you for your good suggestions! I believe that they think either I have it covered, or since I don't have a "real" job, it is my sole responsibility, or both. All of them still went on their summer vacations within the 3 weeks after their mother's stroke! I couldn't believe it! I have gotten some help for a few of the doc. appts. by coming up with "little fibs" that I was unable to do it, or that I have a school function. I'd like to give them the benefit-of-the-doubt, but it's getting harder. I'm working on the job hunt every day...I will then have a legitamate excuse in their eyes, and I wonder how they will handle it??? I know this is venting, and it feels good to know others are listening, for once!

But you don't need to get the job to have an excuse! Tell them now: I am looking for a job. I am frazzled and need some help now to have the bandwidth to be able to look well, and be ready for interviews without looking too frazzled to hire. It is so hard for me to ask, but I am past the breaking point and I really need your help now, as well as later when I get the job my family needs me to have. As "Mom" requires more and more help, we will all have to be on deck more and more." Something like that. When people get too far down the pike of needing help, the request can come out all wrong and judgmental. And it comes past the time you actually need the help -- which was long ago.
I completely understand why they would go on vacation -- from their perspective. Realize the whole family looked forward to it, time away from work was booked (and that is sometimes something you have to request long in advance, especially in summer) and YOU have it covered. Why wouldn't they go? I'm not saying they were right, or that you didn't need help, but just want to help you see the world they way they might be seeing it. The key to all of this is letting them know you CAN'T do it alone anymore. Otherwise, they have no opportunity to change. Good luck!

You might consider private family mediation. I can sense that 8f matters go on as they have, your resentment may build and family get-togethers may become more tense and brittle. Ask mom if she would be amenable to all the sibs sitting down with her and a neutral mediator who would be able to hear everyone and work at identifying needs and interests. Believe me when I tell you that it is far better to talk things out in a safe, confidential manner where a care plan and arrangments and allocatio of duties might be resolved amicably. There is an increasing number of elder care mediators trained and available to help the family.
Doing this now can prevent litigation, bad feelings and hurt for all. There are qualified madiators in most metro areas. The process is open, informal, confidential and ultimately cheaper and faster than lawsuits.

It sounds like they know you are there for her, and take good care of her. They need somewhat of a wake up call. I'd call some kind of get together or family dinner, and just lay the facts out on the table. Tell them you are going back to work soon therefore a plan needs to be put in place now so everyone can meet their personal obligations. Its so wrong how people are self absorbed, and just offer to help their own family, but sometimes you just have to give them a shove in the right direction. I hope they pitch in more. Fair is only fair. At least you know you are doing the best you can in all your doing:)