Several studies note that older siblings have a influence on the drug use of younger siblings than parents have. lesser greater significant negligible

The answer that completes the blank above is the term SIGNIFICANT. There is a significant influence of drug abuse made by the older siblings to the younger siblings as compared to parents. In other words, the effects here of substance use is directly related. The reason behind this is that, siblings are more often together than their parents and most of the time, younger siblings often look up to the older ones. 

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