Grandmother has atrial fibrillation am i at increased risk? i have sinus tachycardia and high blood pressure

Yes and no: It's not inherited but everyone is at risk with aging. At 17, the incidence is near 0. By the time you're 70, the incidence is 10%. Sinus tachycardia is not a risk factor but high blood pressure is! to avoid af when you're older, get your BP under control and keep it there.

Tough call: You're asking a good question that's hard to answer. 3 million people in the us have atrial fibrillation, and of those, ~30% have at least 1 family member with afib. But because there's so much afib out there, and because the prevalence is so high in advanced age (8% at 80yrs, 16%+ at 90) it's hard to decide what the specific risk due to any given family member is.