It is natural to want to help a daughter in your daughter's position, but please take into consideration that you are being a little unfair on your sons. They never ask for help. Probably, they are happy that you help their sister out, but all the same, unless you give them the same, it isn't quite fair.
I would keep very careful account of the money the girl has received and add a codicil to my will, stating that the amount she owes you at your demise is to be deducted from her inheritance.
I would also tell her frankly, that I could no longer lend her money, but that I would help her make out a budget and talk to her bank manager and to social security. If neither she nor her partner are in work, but have children to support she should be entitled to some help.
If her landlord turns her out because she doesn't pay her rent, social security will have to step in.
You are, unfortunately, enabling her to live so irresponsibly by stepping in and helping. I understand why you are doing it, I too would find it hard not to, but in the long run it will be better for your daughter if you stop, now.
I apologise if this sounds harsh, but I have been in a similar position and came to realise that I was doing no good.