This is a very cute idea. Your synopsis introduces the characters really well, but it doesn't give us too much information about the plot. What is the story's conflict? What obstacles do Ella and her baby brother have to overcome?
This blurb makes the book sound scary. Is that what you want to convey? Or is it more humorous? Try to keep the tone consistent with whichever one actually makes up the story. Also, is the sister Japanese? I just wonder about calling her a Samurai as opposed to a Ninja, the latter which implies stealth, while the former makes me think of a Japanese warrior? Very cute idea! I love how mom and dad just see sister patting the baby. Best of luck!