I have been taking care of my niece for the past 7 years, my brother's daugther. The mother is no where to be found, have not had contact with her for the past 2 years. Currently, am not legally her guardian. Considering I don't even know where to start to find the mother, what can I do now to legally get custody?

Where is the father, your brother, in all of this?

He lives with us, but does not provide me with any financial support.

You need to seek guardianship of the child so you have a track record. Then after more time passes you can apply for custody / adoption. How does your brother feel about you adopting the child? Are you single or married?

So the father lives with you. So what do you want to get custody? What do you think custody will accomplish?