All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
If what you mentioned is true and has actually happened, then it is a very strange story that highlights the gravity of having female maids in Muslim homes!
It should be noted that the deeds of a sleeping person are not recorded because he has no control of what happens while sleeping and does not act of his own free will. The Prophet, sallallaahu ‘alayhi wa sallam, said: “The pen has been lifted for three: the sleeping person until he awakens...” Hence, you are not required to repent for what happened.
It should be noted that the child should not be attributed to either of you. In fact, it should be attributed to the mother only because it is an illegitimate child. Moreover, it is impermissible for either of you to acknowledge paternity of the child or accept attributing the child to him if proven to be his biological son.
You are not obliged to marry this woman. However, it is permissible for you to marry her – if you wish – but after she repents from committing Zina and delivers her child.
Allaah Knows best.