What should we allow? (Long)

We are hopefully near the end of our looooooooong journey w/ 3 of our 5 foster kids. They are a sibling group we've had for over 2 years now. We are their only foster placement, so we've been mom and dad since day one. Their bio dad was in jail at the time they were brought into care, and then deported, so he's not seen them in almost 3 years. Bio mom left them alone, (at ages 2,3, and just turned 5) they were 'found', and brought into care........she's not seen them since the day she left them there. Our situation is far from simple! We had done letters and sent pictures to bio dad when he was still in the local jail. At that point, we knew if he wasn't deported, that he'd more than likely get the kids back. However, he was deported and for several months, no one heard anything from him. We do have a relationship here with some of his extended family. They may have sent him pictures and updates, but we have not. In Feb 07, we filed an adoption petition b/c it appeared bio dad was trying to get the kids sent to his country. We knew this would NOT be a good thing for the kids, and considering as how they did not WANT to see their father again, we decided to fight for them. There is a history of domestic violence, drugs, and drinking. We are sitting here a year later, still battling. Bio dad has done NOTHING to get the kids back, other than contest the adoption. He has 2 laywers, one a public defender from our state, the other, a private lawyer paid for by the Consulate's office. Bio dad has not even proven paternity on any of the kids. At one point, we'd been willing to have some type of open arrangement with him, including letters, pictures, and possibly phone calls. However, because he has done NOTHING but cause delays in us adopting the kids, including not sending them a picture, a letter, or anything, our feelings of generosity are long gone. Then add bio mom to the picture. For almost 2 years, she was gone.......no one knew where she was, we just knew she'd taken off with the baby she had been expecting, and made no contact with DCS, etc. She never even attempted a caseplan. Then in mid September, we get a phone call from our CW saying that bio mom was arrested in our area, and they had removed a baby from her. A newborn, not, the one we knew about. Long story, but he'd been in another foster home for about 1 1/2 months before we got him. Before we said yes, we were told bio mom had bonded out of jail and no one had heard from her in 2 weeks. Well, when we picked him up, we were told that she'd been in contact and had shown up to a visit the day prior. Lovely! So now I've met bio mom, and the baby's bio dad (who knew and partially raised my older 3). She has not been very willing to share details about the kids, nor has she given us any pictures. She is still adament that she is going to all 4 kids back. I'm not sure if its her denial, or if her lawyer is blowing smoke...but she will not get the older 3 back....at least not unless something MAJOR happens. She's hidden the 4th child in Mexico. My own personal feelings are not kind toward her, but I must think about whats best for my kids. At this point, I'd say sadly bio mom will get the baby back. The older sibs do not know this is their 1/2 brother. It would crush them to know that bio mom has done visits with him but never even attempted that for them. We know if he is returned, the only hope of seeing him again is to let her see the older 3 and have some type of open adoption. My selfish nature is to tell her no way, forget it, you walked out 2+ years ago, you can't come back now. She's even been living in our city for most of the past 2 years and never made any attempts to check on the kids. However, I want to do what is best for the kids. They definitely see DH and I as their parents. If you ask them who mom and dad are, we are their first answer, and maybe, sometimes, they'll mention their bio parents as an afterthought. The oldest two (now 7 and 6) remember the 'bad' times, where as the now 4.5 year old has no memories, and tends to think bio mom was a saint. So far in the past 2 years bio mom has done NOTHING to turn her life around, and if she does, I fear it will only be long enough to get the baby back, and then she's back to her old ways. Any words of wisdom and advice? I want to make a decision thats best for everyone, not catering only to my selfish desires. Neither bio has or plans to sign an a voluntary surrender, so technically we don't 'have' to offer them anything in writing. Thanks