If you have siblings, you likely know that there are things you can get away with doing to your siblings that you can't do to anyone else. Take this photo for example:
This is me and my cousins, plus my sister, my husband, and my cousin's wife (none of us were married at the time; this photo is OLD).
If you study the photo, you will notice that my sister, in the stripey sweater, is thrusting her hips forward and has her arms behind her back. This is because a second before the photo was taken, I credit-carded her.
Do I go around swiping my hand up random people's butt cracks? Nope. That behavior is reserved for my sister. Poor Cassi.
Your characters will similarly have things they do or personality traits that they can show only around their siblings. Use those interactions to your advantage! Show a gruff character's silly side. Show someone who always acts cheerful dropping the facade around their family.
As a writing exercise, write a scene (even one that doesn't make it into your book) between characters and their siblings. This is a good way to get to know your secondary characters as well as your main character. It'll be interesting to see what it reveals about them!
Need fresh eyes on your query letter? Comment below, and I will select a few random commenters for a critique.
By the way, when you get in the business of critiquing people, you often worry that someone will react badly to your critique and flame you publicly or privately. I've been doing Tuesday Museday for nearly a year now, and I have yet to have this experience. All you OA readers who receive critiques from me have been polite and grateful. I'm grateful to YOU for your classy behavior. Thank you!