Getting started is the hardest part of any project, but especially when writing our own family stories. You might like to look at a sneak peek of a book I am writing to help kids (and adults) write stories. It's FREE on my website at www.bethlamie.com and called Granny's Guide to Fun & Fabulous Family Stories for Kids from 8 to 98. I'd love to know if you find it helpful.
One idea to recall memories is to talk with someone, like your sister, about some favorite event from your life. Then you can write a few lines or a page about what happened, what you felt, what you think now, etc. If you have a tape recorder to use, that makes it even easier to capture a story.
Good luck on starting your stories and remember to have fun doing it!