2 twin 21 month old grand daughters.
Unfortunately you and your daughter are living in a slow crash. The twins at their age do not notice the massive time gap from 5am to 8pm. You are in effect foster parents, and your daughter is in melt down.
In a year or two nusery school will be needed etc. Here in the UK the risk is the children will be taken into care, because there is no proper family unit to support them at present.
You need to talk to your daughter and think of these beautiful children and their future. The dad in this situation also has financial and emotional relationship obligations.
On the emotional front your daughters relationships were not stable and out of this chaos two youngsters have been created. So whether a stable set up can be created that will be workable for the next 18 years+ is the question, because hard decisions have to be made, and these are best made now and not muddled through, because this will hurt these young lives more than you realise. I would also wonder at what kind of mother you daughter is, and what her solution is to the situation. If it is just close ones eyes and hope for the best, that will not work.
I wish you well, but without facing up to what is going on, it is not going to get better, but rather much worse.