We have fostered a baby since birth to 6 months who is up for adoption by adoptive parents who have his 3 siblings.Can we try to adopt him ?

If you have been his/her foster parents for his/her entire life you have very good chances to do it.

Why would you want to separate the child from his siblings?

Is the difference that THIS child was an infant, but the other children older and not as easy for you to deal with their hurt and anger at their biological parents?

Get a lawyer. An attorney will tell you what your chances of adopting this child away from its siblings are.