The differences between the Tucks and the Fosters are more apparent than the similarities, but the two families are similar in some ways. Both families have four members, although the Tuck family has two parents and two children, while the Fosters have a grandmother, two parents, and a single child. Both families have created a system to protect themselves and their home. The Fosters have a "touch-me-not" house that seems to shoo passersby on their way. The Tucks live far off the beaten path to avoid visitors.
To prevent people from discovering their immortality, the Tucks have agreed to see each other only once every ten years. That's their way of protecting each other. Winnie's family protects her by watching over her carefully and keeping her inside the fenced yard.
Both families' members feel strong ties to the other members of their family. Mae can't wait to see her sons, and the sons return home every ten years and enjoy their time together and with their parents. When Winnie returns home, she feels soothed in her own home, feeling the "strong threads too ancient and precious to be broken."
Both families' members support the other members even when they have done something that seems unwise. The Tuck family breaks Mae out of jail and supports her even after her actions killed the man in the yellow jacket. And Winnie's family "drew together staunchly around her" even when she was an accomplice to a jail break.
So despite the apparent differences between the two families, in many ways they are similar, especially in the way they protect, love, and support each other.
You mention that you know what the differences are already, so you need to look at the similarities. They are both in the same area at the beginning of the story. Both families have both parents still involved in the family and both mothers are worried about their child(ren). When you write your comparison, does your teacher want a paragraph or would a Venn Diagram work? You can have one side be the Tuck family and the other side be Winnie Foster's family. Place any similarities in the center where the two circles overlap.
I hope this helps you and that you enjoy the rest of the book. Be certain to read it, there are many differences between the book and the movie.
The Tucks' house is always messy, unorganized, and dirty. The Fosters' (Winnie's family) house is always clean, neat, and organized.
The Tucks: Immortal, eats in complete silence, lives in a cluttered house, laughs a lot, etc.