Op’s husband is neither biologically or legally the father of her eldest.
“A traditional nuclear family consists of a married couple and their biological child or children. A child in a traditional nuclear family lives with both biological parents, if siblings are present, only full brothers and sisters (that is, siblings who share the same two biological parents). No other persons are present in the household (that is, no steprelatives, foster and adopted children, half-siblings, and other relatives or non-relatives).
A blended family/blended household includes at least one step-parent, stepsiblings, and/or half-siblings. A step-parent is the spouse of the child's biological parent but is not the child's biological parent. Step-siblings do not share a common biological parent; the biological parent of one child is the step-parent of the other. Half-siblings share only one biological parent”
https://www.csus.edu/indiv/k/kawamoto/downloadable/50jackson1.htm
“A nuclear family is typically defined as a household consisting of two adult parents—traditionally one male and one female—and their biological children. This family structure has historically been seen as a fundamental social unit in many cultures around the world. While some organizations require the parents to be legally married for a family to be classified as nuclear, others include unmarried couples living with their children. The concept of the nuclear family emerged prominently during the Industrial Revolution, which shifted economic practices and living arrangements, leading to smaller household units as people moved to urban areas for work.”
https://www.ebsco.com/research-starters/social-sciences-and-humanities/nuclear-family
There is, however, this:
Nuclear Families and Elementary Family Types
The nuclear family, sometimes called an elementary family, is often considered to be the idealised image of family: woman as mum and wife, man as husband and father, and two children. In this sense, the nuclear family is presented as the natural or commonsense interpretation of family. However, what constitutes a nuclear family is a source of contention. In general, it remains seen as that idealised image but others such as Golding (2006: 36) have redefined the meaning of nuclear family to include:
single parents, mixed families, stepfamilies, families with biological children, adoptive children, grandchildren, nieces, nephews, [and] foster children.
Although this seems to confuse things from the start, what Golding is trying to say is that the idealised image of the nuclear family can still be formed from the differing family types. For example, if two adults have two adopted children, is this a nuclear family? Regardless, the nuclear family as a family type is still generally considered to be the mother, father, and their direct offspring. Murdock (1949: 2) and Parsons (in parsons & Bales, 1955: 157) saw the nuclear family as a universal component of society meaning that the nuclear family is something that exists in all societies.
sociologymag.com/academic-sociology/subject-areas/sociology-of-family/list-of-family-types-in-sociology/