My mother and grandfather both had osteoarthritis. Does that mean i'll get it?

Not necessarily. Osteoarthritis (oa) is a inflammatory joint condition that occurs usually from overuse, injury, trauma, etc. It is usually not a genetic condition but more an "age" condition. Good joint health (use without over use, keeping inflammation down with ice or medications, and avoiding trauma) will be important in avoiding oa. For good joint health, remember "motion is lotion.".
High predisposition . Age, hereditary predisposition, female gender, and obesity are associated with increased risk of oa. Articular congenital deformities or trauma to the joint also enhance the risk of developing oa. Likely you have high predisposition but i cannot say 100% chances. Consider to avoid foods from animal origen and alkalinize your body (consult a nutritionist knowledgeable on this).