Foster/Pre-Adoptive Feedback/Advice Needed

Hi there, My wife and I accepted the placement of a foster/pre-adoptive 6 month old girl on 12/29/17. She came to us from a recent hospitalization where she underwent surgery due to the latest injury sustained in her birth home. The presented initially with broken arm, then was hospitalized for a depressed skull fracture which required surgical intervention. During this hospitalization they also found a healing rib fracture. Birth parents had no explanation for her injuries. Her 4 and 11 year old siblings were taken on a 72 hour hold following her hospitalization but were subsequently returned as they did not sustain injuries. The parents also had an infant die in their care in 2015 due to co-sleeping. Our SW through DCF stated in her 17 years as an adoption worker, she has not seen a child returned with this level of injury. However, we are nervous with the 4 and 11 year old having returned. We are somewhat confident that the death of the infant in 2015 as well as her injuries shows a recent pattern, but we are nonetheless guarded. We adore her as does our 3 year old son and she is flourishing. We know this will not be a quick process, we are just hoping and praying for a good and safe outcome for her. Our concern is the ability of DCF to prove the inefficiency and safety of the birth parents with the 4 and 11 year old not having issues. We are trying to find solace in the incidents of 2015-now. Birth father also has some assorted charges not pertaining to this and comments have been made implying domestic violence in the home. Birth Dad cannot be in the home with the 4 and 11 year old currently and if he is found present, they will be removed. Does anyone have similar stories/experiences/advice? Hearing some advice or reassurance from other parents that have gone through similar experiences would be very helpful. Thank you!