It doesn't matter how old our kids are, whether they are children, or adults, we still worry about them. The fact that the OP, and her father, has had to support this young family financially, is obviously very concerning, especially now with the prospect of another child coming along.
It's absolutely no one else's choice whether or not they go ahead with the pregnancy, and I personally would never make that suggestion to either of my kids. However, not knowing how bad the circumstances of the the OPs son & DiL are, I can forgive her, perhaps, for telling them that they have choices. They are adults now, and can't be financially reliant on their parents/grandparent forever. Of course, most of us do whatever we can to help our kids, if we have the means to do so, but there has to come a point when they start to fend for themselves, and realize that what they do with their lives isn't someone else's responsibility.
As someone else upthread said, if we'd waited until we could afford to have children, we would've been too old!
I think the best way for the OP to support her son & DiL now, would be to tell them that she's unable to offer extra finances, but to help them find out what financial support is available to them. It's very difficult as a parent to think that our kids are struggling, be there does come a time when they need to stand on their own feet.
Our son and DiL have had some financial difficulties in the past, and we've helped them out wherever we can, but the last advice we gave them was basically that it couldn't go on, and they needed to sort themselves out financially, as they couldn't rely on us every time they had a problem. I'm pleased to say, that they finally seem to have managed it!