The price of private rented accommodation is so appalling in this country, you have all my sympathy.
Living with either your sister or your partner gives you a couple more options and could save you a lot of money and might work for you.
If you move in with your sister, it could be ok but you might resent the situation and her. However, you spend time at your partners and presumably with your four sons. So you wouldn’t be at your sisters all the time. Could you buy something with her that gives you both a bit of independence, perhaps a house that is slightly easier to partially divide into two?
If you move in with your partner, you might do some initial decluttering but your money would stay intact. But again, it might be too much for you.
Retirement flats can be relatively inexpensive, and there are some other schemes where you pay less for the flat or house but you don’t then (I believe) get any money for your sons to inherit when you die.
Do hope it goes well, no easy choices for you, obviously do get as much information as you can and don’t do anything in haste which you might well come to regret. ?