If you put Okalhoma Title IV Eligibiity into a search engine (I generally prefer google myself) you will find several sites relative to information on this ... you might want to determine if these kids are Title IV ... they may already be - it is ruled often by the hearings held at removal from their birth home.
I will note however that I re-peeked at
http://www.nacac.org/AAPchart.html website which details what states cover what and Oklahoma does not seem to be one of the more generous states - they do not offer residential at all if I am reading it correctly and therefore your finalization/adoption assistance (or subsidy) agreement would have to contain a clause to cover any costs associated with medical treatment beyond what is covered by Medicaid including mental health be paid by the State ... and they will balk at it first believe me but if they don't want to relocate these children (and I can't imagine they would but again if they do ... it may be in your best interest as you don't know at this young age what services will be needed in the future and if you can't meet their needs you are doing them an injustice ... know that sounds painful but speaking with first hand experience know that most families cannot afford the $500+ per day residential fee which does not include therapy, education and/or medication costs).
As an additional note - you may also want to do some research on the Voluntary Placement Acts which detail placement into foster care if services are ever required ... this is often activated when the child may need extended services but will be returning home ... in our case it was an uncertaintly ... most likely not a probability and therefore direct care and custody had to be transferred back to the State.
Let me know if you have any further questions ... as said it usually just happens and most sibling groups are considered Title IV ... Title IV for information is just a program that reimburses the state with federal funds allotted for such programs ... if they aren't Title IV you may also ask what needs to be done to have them presented for same for eligibility ... a mound of red tape but the states are usually receptive as it means less $$ from their budgets.