Research confirms a narrowing of families with fewer children and fewer cousins, but it also notes it's more likely for people to know their ancestors:
In their analysis, Alburez-Gutierrez and his colleagues made three major predictions about family structures, also called kinship networks. First, extended family size will likely decrease over time. Second, the composition of families will narrow: Alburez-Gutierrez explains that people will have fewer close-aged relatives in their own generation, such as siblings and cousins, and more ancestors, such as grandparents and great-grandparents. Third, age gaps between generations will grow as people increasingly have children later in life.
https://www.scientificamerican.com/article/shrinking-family-...
Another thing to keep in mind: there is a lot of variability depending on the community and social standards. People in Utah will have a different experience than folks in the Bay Area. In rural northern New York and Ohio, the Amish population has exploded with couples marrying in their early 20s and families typically having at least 5 or 6 kids, sometimes more than 10.
My spouse had our kids in her late 30s, but they have no first cousins on either of the side of the family (of the 6 people in our generation, we're the only two who had kids). Of all of our kids' dozens of friends growing up, only one had more than two siblings and they were an immigrant family.
OTOH, my wife works with people who had kids in their late teens and early 20s and are grandparents by the age of 40, and that's typical in the community.