One parent who is not in grest health at all ( arthritis and other problems caring for another who is immobile)
We have 4 days a week social to help out but it's the shortfall of the other days
This is your probem I think... clearly your immobile parent is not only immobile 4 days out of 7. Where is the help coming from? If it is social care contact them and tell them they need 7 day support. If its self funded then just change it! Is the immobile parent bedridden or just can't do the bed to chair etc transfers alone? I'd be expecting them to at least get a 'get up' and 'go to bed' call from carers. You cannot do this from an hour away.
You have to start looking at everyone as being responsible for their own needs. Your parents, your husband, your teens - and yourself! It is NOT your responsibility to facilitate everone else's life to the detriment of your own.
Your parents need to care for themselves. If they can't then they need to apply for the social support to do so - either social care support folk, or attendance allowance to pay for support, or pay for support themselves. This might take some organising but it's worth it. In the meantime hand one of your days to your brother. Tell him and parents that it is his day - and you will not be visiting. If he chooses not to - they will have to manage. (Yes its harsh but that's reality!)
Your husband and teens need to take their own loads. You work full time you cannot therefore do all the household chores for 6 people. For example make the teens responsible for changing and washing their bedding once a fortnight. Have your laundry baskets sorted by wash type and give them each a day to put a load on, get it out and dry/hung out. Give everone an evening to cook... even the stressed out DH needs to pull his weight as it sounds like you are equally stressed out but because it's caring (womens work?) It doesn't count?
It's hard. But you have to step away and you have to be prepared to tell social care you will step away as currently you are their free resource - which they're not going to willingly give up!