Hi Lottery, tour post sounded very heartfelt and I really think you need to go and talk to someone - a support group, counsellor or similar.
Its incredibly hard when you are surrounded by people but nevertheless isolated.
Can you make some small steps :
No-one really cares about me. I try and give my DS advice and I'm told I'm being miserable (his girlfriend is too controlling and he spends a lot of money on her).
He is an adult and has to make his own life and mistakes. Stop giving advice (unless specifically asked) and just smile and listen.
All the OH is interested in me for is the obvious.
Have a conversation about it. Fill your life with other things, hobbies and join groups. He needs to know he has to give as well as take.
My sister has been pretty unpleasant to me following our mum's death (problems with selling her home).
Let it pass. Grief can have challenging effects on people. If you can, have a quiet undramatic conversation along the lines of time to move on and be friends again.
I just feel totally unwanted for who I really am. I'm just here to pay the bills, do the housework and arrange everything. I want to run away and yet I love DS and OH dearly
Get out and join things, hobby groups, volunteering, gym. You need something to stimulate a life of your own and maybe some volunteering to give you some purpose back?
Most of all, be kind to yourself, value your contribution and take time each day to try and enjoy small things - like a sunny day, a good book, something pretty, a delicious coffee. It builds from there.
The happier you can be in yourself, the better your relationships will become, but it is a slow build of little things rather than a quick fix.