For someone who spent years living in a council flat, the OP seems a bit clueless about how social housing actually works.
Yes, in many places the tenancy can be passed on at least once, but only to someone else in the family who's already living there. The OP wouldn't have been eligible for this as she'd already moved away and got married.
The sister is not paying a "reduced rent". She's paying rent which is calculated to cover the real cost of housing. Social rents usually rise every year in line with the cost of living (linked to inflation).
This rent level is not being subsidised by anyone else; the problem is the opposite: that private landlords charge obscenely high rents because they see housing as a profit-making opportunity, and the housing shortage in many parts of the country means they can get away with this kind of profiteering.
Here in Scotland there is now a rent cap, so private rents can only be increased by a small % each year (6% if the landlord applies to do this, and provides evidence to show why their costs have risen, a max of 3% otherwise).
Those who choose to buy a house, and borrow money to do so, are at the whim of the wider economic situation, and surely understand that any change in interest rates is bound to affect how much they'll have to repay to their mortgage lender?