I have been in the position you find yourselves in - of having a daughter show up on my doorstep pregnant with a small child also. We took her in many times, but there came a point where we could not do it any more. She brought chaos with her, and was only momentarily grateful.
Not taking her in again was one of the hardest things I ever did, but my husband and I are in our 60's. It's been five years, and she hasn't talked to me since. That means I haven't seen my grandchildren and have much sadness, but I don't have the verbal abuse, guilting, manipulation, and all the other things that also broke our hearts. She lied, stole our car - I can go on and on.
Save yourselves. It doesn't mean you don't love her. She has to want to change.
