My boyfriend has 2 small girls and they have been raised by me if it wasnt for me he wouldnt have them now.?

i have raised these girls for 2 years ,one is 2 and other 4 , they call me mom and finacially i take care of there needs , the mother gave up her sole custody almost 2 years ago with the 300 check we gave her , she is not a good mom been in mental hospitals and just had a child feeds and uses marajaunia and loratabs and xanax, she also has been arreasted many times and doesnt have a steady home drags them everywhere she wants to party, the father on the other hand can be a good dad when its convient for him , but also has issues with running . ive moved in to provide stability for the kids because at that time the kids had been staying with there grandma for 2 weeks while father worked and ran, the nother was shaked up with a known theif and drug dealer at time, so im mom and dont want to loose my kids because biologically they arent mine . how do i gain any rights to these girls ,plez help me