Invalid nikaah
Q: I'm from India. 11 years ago my sister loved a Muslim boy who was 2 years younger to her and was a student. When this was disclosed in front of my parents, they asked her to stop all this in every possible way, that is by explaining her the difficulties she will face by marrying that boy, by telling her that if this matter will come out they will loose their respect in the society, by scolding her, by keeping an eye on her, even by beating her. But she and that boy didn't understand as they loved each other madly. Then my sister ran away with that boy and they conducted nikah in front of qazi, a non Muslim boy and a Muslim boy for whom we have doubt that he was of less than 18 years of age (Allah knows the best). No wali was present. Then my parents begged her to come back and they will marry her off with that boy in front of the society. When she and that boy came back, the boy's family said that they will not marry their son to my sister. Our father took us to Saudi Arabia, there he threatened my sister to forget that boy and live peacefully as someone told him that, that nikah was not valid as it was conducted without wali and the witnesses were not fulfilling the Islamic standard. Now everyone thought that, that nikah was invalid including my sister, so we didn't ask for talaq from that boy. After 2 years my sister got married to a nice man with her consent. She is having two kids with that marriage. That man don't know about my sister's past. It's been 9 years now. They are happily married.
Now we are in a big confusion that the previous nikah was valid or not? Is the present nikah and kids valid for her? Is a nikah conducted without wali considered as valid? We did not ask any mufti earlier as we thought that a nikah without wali is invalid. If this is not so then this another marriage has been conducted due to lack of proper knowledge. What is the ruling regarding this matter. Please help us.