Adopting before having biological kids

Our adoptions occurred through foster care, so while we technically had a 7 month old bio when our son (13 mos at the time) was placed with us, he is still the older brother, even though he came into our family after our first bio. It hasn't really effected thier relationship with us or each other that I can tell but they are only 8 years old so who knows what will happen in the future. We also had 2 other bios and adopted another after he came along, so currently our youngest and oldest children are adopted and our bios are in the middle, however I am due in January with our 4th bio, which will make our current youngest (adopted) an older brother to a bio. Frankly, bio vs adopted hasn't really been an issue in any of thier relationships with each other or thier birth order position.