A few interesting facts about siblings and their relative IQs:

1. First-borns in general have higher IQs than later born children. The effect is around 1.5 IQ points on average. Why? IQ scores reflect both genetics and environment. Researchers posit that parents have more time to spend with first-borns on games, learning language and tasks that lead to higher cognitive scores. The presence of older children tends to lead to lesser time and attention for later-born children.  Additionally, there is some evidence that mothers focus less on prenatal care with subsequent pregnancies.
2. Siblings born closer together have more similar IQs than those born more years apart.  This result is reflective of the nurture/environmental aspect of IQs. Siblings close in age share similar experiences and likely have similar socioeconomic upbringings as compared to siblings born further apart.
3. A 2014 study out of Russia found that lower IQ older siblings are more dominant and antagonistic towards their younger siblings and that higher IQ older siblings are more cooperative with their younger siblings.
4. Genetically, intelligence and IQ is polygenic meaning that many genes are involved in determining intelligence.  As such, the correlation between parents and their children and among siblings can vary within a pretty decent range.  A study from 1979 found that the correlation between sibling IQs was 0.49. A researcher at Stanford has reported the following correlations:

• Same person (tested twice) .95
• Identical twins—Reared together .86
• Fraternal twins—Reared together .55
• Biological siblings—Reared together .47
• Unrelated children—Reared together .30
• Parent-child—Living together .42
• Adoptive parent – child – Living together 0.19
• Identical twins—Reared apart .76
• Fraternal twins—Reared apart .35
• Biological siblings—Reared apart .24
• Parent-child—Living apart .22

Refresher on correlations:  In the results shown above, the closer the number is to 1.0, the more similar are the IQs.  So if you and your sibling score a 1.0, then you have the exact same IQ.  A 0.0 means you have as different an IQ as possible.