Need advice and I doing the right thing

Here's my story and it's a long one so I will try to shorten it as much as possible me and my wife took in our neice 7 and nephew 2 through the foster program and have also gotten our licenses to foster it was supposed to be short term until my sister could get back on her feet and her husband was in drug rehab they split because of a abusive relationship on both parts not just him abusing her there was drugs and violence on both sides now fast forward over a year and we still have the 2 kids now 8 and 3 the father has gotten out of rehab and is doing really well but he has pending charges against him that if he is found guilty he will serve time and they will terminate his rights my sister on the other hand hasn't done a fraction of her service's to get the kids back me and my wife told the social worker we would adopt the 2 that's living with us but here's the twist there is a grand total of 4 kids my brother took in the other 2 but he has no plans to adopt and I don't have room for 4 kids or the financial stability for all 4 the 2 my brother have are both boys 9 and 5 the oldest has behavior problems that therapy has not helped and the 5 year old is starting to take after his older brother And picking the same problems am I doing the right thing by separating the 2 I have from the other 2 if I adopt them because the other 2 will most likely go to a theraputic foster home or possibly be adopted out my real concern is in the future will the 2 I adopt resent me because I couldn't take there siblings