Should I focus on Bokmål or Nynorsk if I want to sound natural with my Norwegian relatives?

My family is from the Vestland region and they use a lot of Nynorsk-leaning vocabulary, but all my learning materials are strictly Bokmål. I want to sound more natural when I visit them this summer rather than like a textbook. Is it worth trying to learn Nynorsk grammar rules now, or should I just stick to the standard and hope they understand my 'Oslo-speak'?