The Mishna (Ketubot 12:1) in today’s daf (Ketubot 101b) describes a situation where a man marries a woman with a daughter from a previous marriage and commits to financially sustain her.
Significantly, as Rabbi Bleich explains in his essay titled ‘Support of Non-Biological Children in Jewish Law’ (see https://bit.ly/3D0eycB), ‘the Mishnah does not posit a requirement for kinyan (i.e., an overt symbolic act indicative of determination to consummate a transfer of property or assumption of an obligation)…[This is because] Maimonides rules that marriage itself serves as kinyan for the groom’s undertaking to support his wife’s daughter. The underlying concept is reflected in common law in its recognition of marriage as consideration supporting a contractual undertaking’.
On this basis, he continues to explain that ‘it may readily be concluded that Jewish law enforces an obligation to support an adopted child where the obligation is undertaken explicitly. As to instances where there is no explicit undertaking, the late Sephardic Chief Rabbi of Israel, Rabbi Ben-Zion Uziel, in a compendium on adoption included in his Sha’arei Uzi’el, states that none is required for the parent to become halakhically liable for the child’s support. He offers only the cryptic explanation that parents who have entered into an adoptive relationship are regarded as if they have expressly declared that they obligate themselves jointly and severally to all of the obligations of parents vis-à-vis their children.’
Today, many children are being raised in blended families, and different situations can arise relating to the financial responsibilities of a parent towards their step-children. And what we learn from our Mishna, and especially from the remarks of Rabbi Uziel, is that when two people who have children from another relationship choose to marry, the act of marriage brings with it obligations to those children as well.