First of all, in Colorado, the term is not custody it is "parental rights and responsibilities".
You do have rights, and they are constitutional, to parent your child. Those rights, however, will be balanced against the right of your child to have his best interests protected.
If you are truly willing to step up and be in this boy's life, I would make many suggestions. First, get involved in the Dependency and Neglect case by entering an appearance (you may want to retain an attorney to help you) and by asking the court to let you have a relationship with your son. Second, contact the sister and ask to be introduced, and to gradually get to know your son. Third, get some good books on child development and parenting, so that you will understand what it is that you are going to be doing.
No court is going to give you a child who, at age two, doesn't know you at all. This would be traumatic for the child, and you need to understand that and to understand why it would be traumatic.
But, if you get to know your son, and if you establish a good relationship, then gradually a court would likely give you increasing responsibilities and time with your son. If the mother really cannot handle being a mother full time, then it is possible that you could step in as the full time parent. However, two years of age is a really critical developmental time, and it is unlikely that a court would let you step in overnight for that reason.