Edge26 you are being "Abused" by your son.
Some lovely replies already and some good advice but I really don't think you are equipped or prepared to follow the good advice YET.
IF, I stress IF, you want to Take Better Care of Yourself then there are a number of things you need to do in order to gain the Skills and Techniques you will need to take that better care of yourself.
The "things" are mostly about learning and then practicing the What to say and the How to say them.
If you want to be better prepared to address your son's behaviour start by researching what counts as "Abuse" in its differing forms. There's lots of good information and help on the web.
Once you gain some clear understanding of "How" he delivers the abuse - you've said he is verbally abusive - "What Effect" his abuse has on you - you are upset - by his Bullying.
You already understand both these and you have clear understanding of "What" he wants to "Get" from his Emotional Blackmail - more childcare.
So you've made a good start by having clarity of "What" he is doing, "How" that effects you and "What he wants to get out of "Abusing" YOU.
IF, you want to have better Defence against your Abusive son once you're more informed around the forms of abuse then you need to move on to Assertiveness Training.
Research, read, read until you have a good understanding of what Assertiveness is and How to use it.
If you have a friend who you could do the work with that's good because once you've got the understanding you need to practice, practice, practice what you've learned.
If you can find a course or afford a Counsellor to support you through this work even better.
You are 1 caring person up against 2 adults who want to maximise your Usefulness by any means.
You - very reasonably - want to be Useful but your inner self wants to be Respected and Valued.
I used "Useful" and "Valuable" a lot in my work as they are simple small words that allow people to analyse and evaluate "How" they are treated by others, "How" they gave Permission for the relationship get into its current state and most importantly "What" the person can do to change things.
I used to ask clients to start by "Being Valuable to Themselves". If we only see ourselves as "Useful" others will too.
Even when we understand ourselves as "Valuable" others may not but at least then we are clear and can start making "Choices" as to "How" we take care of ourselves and/or move the relationship forward.
I may be wrong but I read your post as coming from a person wants to make changes in the transactions between mother and son.
A person who instinctively knows she is being abused and bullied but doesn't - AS YET- know How to take care of herself OR even if she is being reasonable in wanting to have a say in her own life.
You have the Right to Your Life. Being Helpful (Useful) to you children and GC is what you are doing. Bullying you to get More from you is Wrong Treatment of you but only you can change things.
I wish you the best possible outcome and I sincerely hope you do the reading, gain the knowledge and practice the skills you will need to take better care of yourself because sadly your son and DiL will not change their behaviour towards you. They hope to gain too much from you -Free Chid Care.