How to make her real to them again? Long

I'll try to keep this as short as possible, but for those who don't know the history, I'll fill you in. Guessing it will be more long than short.We've had 3 of our 6 kids for 2.5 years (full bio sibs). They were just turned 5, 3, and 2 when we got them. They are now 7, 6, and 5. In the fall of last year we got a call from their CW saying that their infant brother had been taken in to care, were we interested in a placement.We said yes and its been :hypno::hypno::hypno::hypno::hypno::hypno: ever since!!In the 2 1/2 years we've had the children, they've had no visitations with either bio parent, as bio mom was MIA and bio dad was in jail, then deported. At this point the children do not know that the baby is their bio brother. He is 1/2 brother through bio mom, and had weekly visits w/ her and his dad.The kids love baby and he's crazy about them. We haven't really talked about 'his' parents and they don't really seem to question why they don't ever go to his visits w/ me. In early April, we worked out an open adoption agreement with bio mom. We didn't have to, as the case against her was pretty solid. However, for their sake, we didn't want to cut her out of their lives completely. There was no abuse, more just neglect, drugs, and a lot of bad choices. Plus we knew the likelihood of the baby being returned to her was pretty high and we selfishly wanted to know how he was doing.The agreement is for 2x per year with several contingents and clauses. The first visit will not be held until at least 4 months post finalizing the adoption. Since we are still fighting their bio dad in court, we could be looking at the first visit being toward the end of the year, if not early next year. In Jan, it will have been 3 years since they've seen her.Throughout the time we've been in contact with bio mom, she's given us some gifts for the kids and pictures. We've shared the pictures with them, but have held on to the gifts for now. There is nothing wrong with them, but we were waiting to make sure she held on to getting presents for all their bdays. The youngest just turned 5 last week, and she did bring a gift, so we have decided to give them all the stuff this weekend.We are going to make her some pictures for mothers day, with their hand prints, a picture, etc. They know we've talked to her at court, but the judge has said she's still not allowed to see them yet. They also know that one day we will see her again though.We look for the baby to go home sometime in July, right around his 1st bday. He's just now started home based visits. They also have a 1/2 brother (full to baby) who will turn 2 in July. They don't know about him either.One of the stipulations in the agreement is that bio mom has to bring all current and future siblings to the visits. Our goal is to be able to have an open relationship with her so that all the kids can know who each other are and have some type of relationship. Since the baby will be going home before we finalize the adoption, we know we need to tell them that he's their bio brother and who he's going to go back to. I'm sure its going to be really hard for them to understand why he gets to go back to her and they don't. She never attempted anything, including visits, for them, and while they don't understand all of that, they do know she wasn't here. Any advice how to start preparing them now? DH thought maybe start with some phone calls. First step we know will be the gifts and letters she's given recently. We want this relationship to work and we know there will be plenty of bumps and bruised along the way. We want to prepare the kids for seeing her within all of our new roles. It was super easy for them to attach to DH and I, and our family since there was no contact with anyone else in their bio family, except some cousins they didn't remember. However i know there will be some confusion for them how to interact with her.Thanks for hangin' in there and reading all this. Such a tangled history here. Five months ago, we would have said she could never see them again, but hearts all around have changed.