Specifically, it's not really about kendo but it's about an older step sister who had adopted a younger step brother. Both have feelings for one another.

The younger stepbrother then encounters another older girl who has a guard of her own and the older girl fell in love with him.

Chiguhagu Lovers? (For future reference: searching for "kendo" and "stepsibling love" with the advanced search yields two results... the other one is a hentai)

~4400 manga completed
~1700 ongoing...