I think it will depend on the situations of the children in the sibling group. Statistically they say that groups of more then two siblings are more sucessful then two and no one really knows why expect perhaps the bonding between the children is stronger. The most difficult sibling groups to adopt are two children and families who do adopt two have the highest rate of disrutpions...???
My personal concerns would be about the baby you now have... and is your 17 year old going to be off to college due to the chaos and stress--like my older two were? I would NOT count on your older daughter to stay around and help you..... My older children were both motivated to move away and personally have decided to delay marriage and parenthood due to the expereince of the adoption of our two little ones (YIPPY!!!)
The big question will be what issues a sibling group may have and the safety issues for the baby.... That vast majority of us who do choose to adopt older/or sibling groups have come to an agreement that it is always best to have our biological children older and have a good distance between the ages...in order to insure the lifetime safety of our biological children.... I know this might sound insensitive but parents who adopt a child who ends up to have RAD or possible sexual abuse are on a long path and it is stressful...and it can be so hard... I cannot even imagine adding to this feelings of anger should an adopted child harm the biological baby in any way...
The mojority of siblings adoptions are complex we only have two and the weekly appoinment for therapy, doctors and special education are crazy only with two....Next Thrusday at 10 a.m. I am supposed to be in three different places at the same time
Half of the time one doctor or therapists wants to see one child without the other interfering...and half the time I seem to scramble for obne arrangement of another.... Monday my son has early intervention class from 9-10 sister cannot go--but at 11 sister see the shrink who has asked me not to bring brother---so I am having to pay my niece to ride in the car and watch one child for ano hour at a time in a parking lot with one kid at a time EEEEEK!!!!!
Some days are just completely NUTS.
Honestly the reality is that the majority of sibling groups will include at least one high maintance child.... and some children have some significant issues that go unnoticed or detected or maybe even covered-up until they have had time to be in a family for awhile and you end up realizing there are issues...
I want to help in our daughters 1st grade class but---our son has to have day-care if I do... I need to help in her classroom because with 26 students the teacher does not catch the little things that need to be caught....
Our daughter is mean to her own biological brother...she has bitten him, pours sand on his head in the sand box--in his clothes...takes his toys breaks and hides them... and has to be watched at all times when her brother is up...
When our children were placed nearly two years ago NO problems were known.... Today we have diagnosis of possible Bi-polar and ADHD with our daughter and RAD (reactive attachment disorder) and Post traumatic stress disorder...three different medications... We got our son three days before he turned one... he was diagnosed last year as being a full 10 months developmentally delayed....cannot talk yet and has significant issues.... I can deal with all of this and love these kids with all my heart but I would feel guilty if I had to sacrifice time to a biological child in order to do all the work I need to do with these children....
My recomendation might be to consider the adoption of a single older child like 10 or older to start off with.... This is old enough to know if things are starge and young enough to have time but still gives good distance between the age of your biological child. YThen later if you feel things are working well I might consider a sibling group....maybe after your youngest has the ability to communicate and let you know if she faces problems...?
Just my oppinions.... of course.... good luck and I happyy to see that you are considering all the implications of such an adoption.