Allah Says in the Holy Quran Chapter 4 Surah Nisaa verse 23:
23 Prohibited to you (for marriage) are: your mother, daughters, sisters, father's sisters, mother's sisters, brother's daughters, sister's daughters, foster-mothers (who gave you suck), foster-sisters, your wives' mothers, your step-daughters under your guardianship born of your wives to whom ye have gone in, no prohibition if ye have not gone in; (those who have been) wives of your sons proceeding from your loins, and two sisters in wedlock at one and the same time except for what is past; for Allah is Oft-Forgiving Most Merciful.

If a man has a son from a previous marriage, and happens to marry a woman who has a daughter from another marriage..the son of the man and the daughter of the woman (or vice versa) are non-mehrams to each other and it would be absolutely lawful for the two to seek to marry each other in Shariah, if indeed they wish to do so.

But lets say a man has a son from a previous marriage, and marries a woman and Allah blesses them with a daughter in their marriage.because the son (from the previous marriage) shares the same biological father as the daughter from the present marriage..they would be considered mehrams to each other in the Sight of Shariah Law, and it would be absolutely unlawful for the two to seek to marry each other.