Is a Muslim woman allowed (in Islamic law) to feed her baby in front of her step-father? Does she have to put Hijaab in the presence of her step-father, as some say 'yes' and some say 'no'?

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.

A stepfather, who has already consummated the marriage with a woman who has a daughter, becomes a Mahram to this daughter (his stepdaughter).

Allaah Says (what means): {Prohibited to you [for marriage] are your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters, your brother’s daughters, your sister’s daughters, your [milk] mothers who nursed you, your sisters through nursing, your wives’ mothers, and your Rabaa’ib (step daughters, plural of Rabeebah) under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you. …} [Quran 4:23]

Ar-Rabeebah or Ar-Rabeeb (a stepdaughter or a stepson) is named so because a stepfather takes the responsibility of educating her or him exactly like his own daughter or son. There is a consensus of opinion among the scholars that consummating the marriage with a mother prevents marriage to her daughter.

Thus, you are not required to wear Hijab in front of your stepfather just as you are not obliged to do so in front of your Mahaarim (plural of Mahram), i.e. you may unveil your head, hair, neck, arms and so on.

Imaams Al-Baydhaawi and Al-Baghawi said: ‘‘Stepfathers can see of their stepdaughters what is usually unveiled while one (a woman) is performing daily chores.”

Therefore, you should know that it is forbidden for you to show your breast, belly, back and the like in the presence of your stepfather. This is to prevent every means that may lead to temptation.

The Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever grazes his herd in the vicinity of an exclusively reserved pasture, it is most likely that some of his animals might get into that pasture.” [Muslim]

Allaah Knows best.