Yes, the "financial support" puts a different face on it, IMO. I know you and DH love DD and may want to help her out. But if she and SIL ask you to give/loan them any money or cover any expenses, I hope you two will only do what you can w/ willing hearts (and what you can afford, of course). If you can't/don't want to give them what they ask (if they do), please be prepared for the fact that they may pull away again (sigh). I hope not, but I'm afraid it's possible.
Beyond that, clearly, you know SIL even though DD didn't tell you directly that they are married. And it sounds as if there have been issues between you/DH and SIL since before he was SIL. Could that be why she didn't let you know personally that they were married (it seems she told you indirectly via your other DD)? If so, maybe that's why he'll be there - b/c he's part of the issue. I wish you could meet w/ just DD, at first, but oh well.
"On the other hand, I just want to see her and hug her, let her know I love her. Try to re-build."
I think you definitely should hug DD and say that you love her. But, IMO, rebuilding will depend a lot on how the conversation goes (which gets back to why you're nervous, I guess). I may be wrong, and those who have been through this, like Smileless w/ her ES, may say otherwise, but I think you and DH should listen first and see what direction DD and SIL want the conversation to take. That will give you and idea of why they agreed to meet in the first place. Is it just to vent? Seek help w/ finances? Reconcile? I hope it's mostly the last one, but you'll see, of course.