I am a US citizen I am marrying a woman from Domican Republic how can i bring her children here?

You can sponsor your fiancé for a k1 fiancé visa and your future step children as k2 derivative beneficiaries. The visas would be processed at the overseas consulate and when the visas are available, your family can enter the US but you must marry your fiancé within 90 days , and then they all can adjust status to obtain their green cards.