A woman says that she has a stepson and does not know whether he was born from a legal or an illegal relationship. The question is: if this stepson was born from an illegal relationship, can she take off her Hijaab in front of him? Is it permissible for him to be her Mahram (permanently unmarriageable) during travel or in a journey of ‘Umrah or Hajj?

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.

The basic ruling is that a son is attributed to his father because the Prophet, sallallaahu ‘alayhi wa sallam, said: "The child is for the (owner of the) bed." [Al-Bukhari and Muslim]

If it is proven by Sharee'ah-approved evidence that the son is not attributed to his father, such as the father denying his fatherhood to the child by means of li’an (accusation of adultery towards his wife without witnesses), or that the child was born from a marriage which was unanimously declared invalid or the like of what prevents attributing the child to that father, then that stepson is not a Mahram to that woman. Thus it is impermissible for him to see anything of her except what a non-Mahram man can see. Also, it is impermissible for her to travel with him without a Mahram.

However, if that is not proven, based on Sharee'ah evidence, then the basic ruling is that he is a Mahram to her.  

Allaah Knows best.