Adopting a Toddler

I know adopting a toddler is not really consider an older child adoption but I was still hoping for some advice here. We have been presented with a possible situation with an almost 3 year old. In our home we have 2 bio kids, a 5 yo girl and a 3 yo boy. With our son and the potential child being only 5 months apart, are we setting ourselves up with serious sibling rivalry and competition between the two? I'd really be interested in those who have tread this territory and what your experience has been. The child is still living with the mom and is not in foster care and social services in not involved. The parents split up in the last couple of months and the mom doesn't think she can do it all on her own. Thanks, Tammi