The books make it seem its a hereditary trait with Master Raymond being the "Father" all time travelers are descendant from and Claire's daughter and grandchildren having the trait and the ability also in Roger's family. Does it skip certain generations? It's it only when there's a threat to the time lines? It doesn't make sense that Claire's parents couldn't but maybe more likely they didnt know since they never came across a stone circle?

If I recall correctly, Frank was friendly with Roger's uncle and had visited him so he could have gone to the abandoned Lallybroch and left the letter. Don't forget he placed the gravestone with Jamie's name for Claire to find and know he survived the battle.