I’m taking this as face value and looking at practical ways the family could help GD1.
To answer RosieMaw’s question. From reading both threads, I believe OP said GD2 had been saving since her teens. Although she only works part time now due to mental health issues and earns about £9,000 pa that doesn’t necessarily mean she hasn’t, at some point, worked full time. Key is that she has not contributed anything to the household. Over say fifteen years from age 16 it would be possible to save at a rate of £5000 or more a year. I believe GD1 also has savings but has contributed housekeeping.
The crux seems to be that neither woman could afford a place of their own until now but now that GD2 has a boyfriend, that gives the two of them the scope to buy somewhere place to live whereas on her own, GD1 cannot and seemingly never will be able to.
I was thinking about possible solutions to this. OP says (on the first thread) that she visits her son’s house at least three times a week. That suggests she lives very close by. With a granddaughter almost 40, I’m thinking OP must be around 80. OP doesn’t mention a husband so I am assuming she is divorced or widowed. She needs to be thinking about her longevity and future care needs.
We do not know if the parents and OP own their own homes. If they do then there is scope for ordinary inheritance in the future.
If they are tenants in local authority or housing association properties, then there may be inheritance rights too. It depends on when the tenancy agreement began:
Before April 1 2012
You have succession rights if you live with your relative for at least a year immediately before their death.
On or after 1 April 2012
You have succession rights if the tenancy agreement says a close relative can succeed, and you meet the conditions in the agreement.
You may have to show you lived with the tenant for at least a year if you have succession rights.
A grandchild can inherit a tenancy.
If you inherit a property and it has more bedrooms than you need, you may be asked to move to a smaller property.
england.shelter.org.uk/housing_advice/council_housing_association/can_you_inherit_a_council_tenancy
Various permutations come to mind about how to give GD1 a bit of help toward having a home of her own including how to utilise the spare bedroom that GD2 will leave. Maybe it could be rented out to raise some extra cash to help her.
If OP has a tenancy, would it be possible for GD1 could move in with her grandmother to inherit the tenancy when she dies? I think an arrangement of this kind might go some way to compensating her for the unequal treatment the two daughters have had over so many years.
Just some lateral thinking. It doesn't help resolve the current tensions but might give GD1 a bit of hope for the future.