How is Sammy Johnstone important to the play Blood Brothers?

Blood Brothers by Willy Russell tells the story of struggling single mother Mrs. Johnstone. Soon after her husband abandons the family, she discovers she is pregnant with twins. She cannot afford to raise two more children, so she gives one of the babies to Mrs. Lyons. Mrs. Lyons is from the upper class and employs Mrs. Johnstone's cleaning services. Mrs. Lyons has had trouble conceiving, so she convinces Mrs. Johnstone to give up one of her twins. Edwards is raised as a Lyons, while Mickey grows up in the Johnstone household.
Sammy Johnstone is one of Mickey's seven older siblings. Sammy is a trouble maker and therefore is a bad influence on Mickey. Sammy is important because he involves Mickey in an attempted burglary that turns into a murder. As a result, Mickey ends up in prison, which leads to depression and addiction to pills.
At the climax of the show, Mickey retrieves the gun that Sammy has hidden under the floorboards. Mickey shoots Edward with the gun and then is shot by police.
Sammy is a minor character, but his actions influence Mickey's trajectory. Additionally, Sammy serves as another example of what can happen to a child growing up in poverty, not afforded the luxuries that a child like Edward has growing up.
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