We are born into our families and so have no choice about who they are and how they behave.
It’s different for our characters as we get to create their families.
That is your task.
After you choose a name and age for your character, then fill out a family tree.
You probably should search online for a model. This will give you a working model.
Start with the grandparents on both sides. Name them. Name the people they married. Who were these people? What jobs did they have? Were they trustworthy or liars? Did they own their home or rent? Where did they live? In a city or in the country?
Move to the next generation. Include all the relatives at this level, which means the all the siblings. Answer the same questions as you did before.
That takes you to your character’s generation. Your family tree should be expansive at this point. Answer all the questions for each person along this line.
If your character is married and has children, list the details for each.
This is a time-consuming task, but the effort will pay off once your story begins.
Have fun with this one!