Gilly - what a life! I think there will be many on here who can identify with that from various stages in our own lives. But it does sound as though you truly are beset on all sides.
Is there a way of getting various members of the family together to try and sort this out? One of the difficulties is that we, as mothers and gmothers, tend to make ourselves freely available to all and find it hard to say no - for lots of reasons I think - sometimes even because we like to feel needed and it makes sure we have a role at a stage in our lives when we fear that might begin to slip away soon.
Maybe my own current experience might be relevant. If not, I apologise for bringing this in.
I am in the middle of a situation where I am having to sit back and let others take care of me and it feels very hard as I have always felt (rightly or wrongly) that I had a pivotal role in the family. I am discovering that I don't - at least not quite in the way I had thought. It is a salutary, but painful, lesson - letting go and allowing others take responsibility for their own lives, and for them also to enjoy being able to help me. Circumstances have forced me to face this, and it is not so bad! I think it will change relationships in the family, but that is how life moves on.
What is very clear is that you cannot continue as you are, and, as long as the family believe that you can, then nothing will change.
This conversation started on the subject of sex and relationships but it is clear that the problem is very much wider.
You could possibly look at each of the roles that you have in the family and at work and sit down with your OH and think about how these needs might be met in a way that did not result in you being the one who bears the burden. There will, I am sure, be ways. And some of those ways will involve them taking on their own responsibilities rather than you finding yourself doing so. It might involve outside agencies like nurseries or childminders - my wee GD, who has been coming to me one day a week (prior to my being unwell) also goes to a nursery one day and a childminder another - she gains hugely from this variety of experience. Maybe your family need to look into the financial help available for these.
You cannot be the sole "entertainment" for your Mum and Dad - maybe you could all research what other options there might be in the area, and put this to them.
You cannot be the one who bears all the burdens - not only is it not fair, but it might possibly not be the best thing for these young people who need to take responsibility for their own lives.
It is hard isn't it? I wish you well with it all.