The person in question is actually Mukti not Devki.
Mukti was Prerna's foster child. When Prem came back to Prerna, he and Mukti fought a lot. Then Mukti fell in love with Prem. One rape and five years of denial later, Prem realized he felt the same way. He proposed to her but she was engaged to another guy. On her wedding day, Prem grabbed Mukti and forced her to elope with him. However, she told him that she still loved him and was marrying her fiance only because Komolika had kidnapped Mr. Bajaj and was forcing Mukti to marry anyone except Prem.
When Prem went to Komo, she confirmed this. He took her back to the mandap.(the second video you saw). Then, AP reveal that Bajaj was never kidnapped by Komo so Mukti should go back to Prem, who had genuinely become "good".
Mukti ran back to Prem (the first video you mentioned) and died shortly afterwards.
We never found out who killed her though both Komolika and Debonita(Komo's niece who was obsessesed with Prem) are asummed to be guilty.
Her look-alike, Devki, came a couple of months later to help him recover from the loss. She, too, left him after spending a night with him because she had blood cancer. She has just now returned.
The videos for Prem-Mukti-Devki track are available in Main Prem Ki Deewani Hoon sticky.
Hope that helped😉
who raped mukti?