Have you considered asking the children's mother to do a parenting course like Incredible Years? I was a 'volunteer grandma' on one for a while (it was free for participants) and the parents had a lot of fun, it's not judgemental in any way and there is separate care for pre-schoolers (at my branch anyway).
Please read about it below then ask your local CAB who runs it in your area. There may also be other parenting courses too.
To ease your burden, ask about Barnardos who used to have, and maybe still do, in-home help for people caring for children and who are finding it a strain.
Your eldest gc is just becoming a person in her own right and aware of Mum's shortcomings. She's probably genuinely concerned about her mum but feels helpless to do anything about it.
Have you spoken to her about the fact that Mum might need a little help as she gets so tired and worried (age-appropriate phrases ) and can't always think straight?
I have found it is best to be honest, rather than hedge or think children cannot understand.
I'm afraid I don't have much faith in Social Services; they are so stretched and have a Prioritising Scheme, whereby the worse-case (violence and sexual abuse) come at the top, and happy children with loving grandparents are way down the list.
The other thing is - your stepson, whe he comes into his fortune may be able to help with a day-nanny service to give you a break.
Looking after 3 is tiring and he may be prepared to finance this, one day a week or more. There are many agencies who send out people to help, or have sole care.
Depending on your circumstances, you may also get some sort of benefit towards this.
You are not in any way selfish, quite the contrary but clearly you are almost at the end of your tether. So I hope some of these ideas help.
www.dailymail.co.uk/health/article-63973/How-The-Incredible-Years-parent-training-programme-works.html
www.barnardos.org.uk/what-we-do/helping-families