How can I include my brother and sister with what my relative left me?

You could disclaim (renounce) some of all of your inheritance. Unless the Will provides otherwise, the disclaimed property will go as if you had predeceased your relative. If it would then go to your children, they could also disclaim. If you have grandchildren who would then take, they could also disclaim. It is often possible to obtain court approval to disclaim on behalf of a minor. Keep in mind that the due date for a disclaimer is 9 months from date of death (or 9 months after reaching age 21, if later), and it takes some time to prepare the necessary papers (and to obtain court approval for disclaimers on behalf of any minors).