Adopting a child abroad /Adoption Visa

Neither of the first two attorney answers are completely correct. In July, 2014, the Uniform Accreditation Act took effect, which means that every single international adoption, even relative adoptions, must be conducted by a Hague accredited adoption entity. Usually this is an adoption agency. Immigration attorneys used to be able to do relative adoptions but they cannot do this work anymore.
Fortunately, Colombia does in fact allow international adoptions, so you have that going for you. Google Hague adoption agencies and Colombia adoption programs, and you can start interviewing agencies.
It will be a paperwork intensive process, but every child needs a safe and loving permanent family. Good luck!

Those "steps" as you call them are far too many and take a long time to execute each, but here are some highlights:
1. Formally adopting your niece according to pertinent Colombian adoption law in place; as well as,
2. Proving legal and physical custody of the child for at least 2 years prior the the child having attained age 16;
3. Only then launching the US immigration process.