Far be it for me to suggest how you should juggle all of your responsibilities, as you seem to have many important things on your plate. Perhaps you can devise a rotation schedule for yourself, deciding whom you see and help and when. Keep in mind that sibling rivalry does not end when children are grown, and that one or more of your children may become jealous if you and your husband spend more time with one of their siblings.
There is no easy solution to your predicament, but you need to put yourself first or you will not be of any help to anyone. Consider choosing between Shabbos and Sunday to help or visit with your children. Also, if you have siblings who can help with your parents, ask them to do so.
In sum, you must make time for self-relaxation. And if you say no to your children with love and warmth, it will be easier for them to swallow the new reality. Also, if you can afford to do so, offer to pay for a babysitter. This will serve a dual purpose: it will help your children feel taken care of, and you will no longer feel overworked.
Your children are fortunate to have such a special mother and mother-in-law. Hatzlachah.