Is a private meeting between spouses considered consummation, and does the fatwa of Imam Ibn Baz, which prohibits a girl to her mother's husband merely by virtue of the marriage contract, apply? And in this case, is the questioner considered a stepdaughter to him or a stranger?

The wife of the father becomes forbidden to his sons simply by the marriage contract. The mother of the wife becomes forbidden to the husband simply by the marriage contract. As for the wife's daughter (stepdaughter), she does not become forbidden unless the husband consummates the marriage with her mother. If he does not consummate the marriage with her, she does not become forbidden. However, if consummation occurs, the daughter becomes forbidden.

Scholars have differed regarding the ruling on seclusion with the wife without intercourse, concerning the stepdaughter: 1. The Hanafis and some Hanbalis held that seclusion is like intercourse. Thus, if a man secludes himself with his wife, her daughter becomes forbidden to him. 2. The majority of scholars held that seclusion does not make her forbidden. Therefore, the daughter does not become forbidden unless the mother is intercoursed with. 3. Malik, some Hanbalis, and some Shafi'is held, in one opinion, that if seclusion is accompanied by looking with lust or touching with lust, then the prohibition occurs.

In summary: Seclusion accompanied by lustful touching renders the stepdaughter forbidden according to the majority. However, if the seclusion is without lustful touching or looking, it renders the stepdaughter forbidden according to the Hanafis and some Hanbalis.