It is my understanding that sibling scores tend to be quite close. If I find a study which mentions the typical point spread among siblings, I will post it. Meanwhile, someone else may post it.
That said, a family may have children with different birth experiences, health issues, or genetic conditions which may greatly influence brain development.
A family could also undergo a tremendous change which may impact the family focus on education, amount of time, attention, and support available to encourage some of the siblings. For example, any of these life-altering events or combination of similar events: death of one or both parents, loss of support system such as grandparents, aunts, uncles, job loss, health crisis, incarceration and/or legal issues.
Some families may be immersed in a culture which emphasizes education for males but does not value or encourage intelligence in females.
IQ is often to said to be comprised of Fluid Intelligence (native ability for logic and reasoning) and Crystal Intelligence (what one has learned through education and/or experience).
Additionally, scores on different test instruments are not necessarily equivalent. This chart lends some insight:
https://www.hoagiesgifted.org/highly_profoundly.htm
