Yes. The prevalence of adhd in the general population is ~5% but it goes up to 18% if one of your parents have it, 35% if your sibling has it, and 97% if your identical twin has it. Best to avoid nicotine and alcohol in all forms during pregnancy to diminish the risk of penetration of the disease (actually getting what you've inherited).
High likelihood. There is an inheritance relationship between parents with ADHD.
Maybe. While many say a adhd is genetic, i think it is the actual underlying medical problems that are genetic. Allergies and low blood sugar tend to be genetic and can cause one to look like they have adhd but they really don't. When the medical problem is corrected the adhd symptoms often go away.