Warm greetings @Catharine- I’m so sorry for your loss 
@jan_TheGreenWitch said it perfectly- I completely agree that any members who have a place on the branches of the family tree can count as “ancestors” 
I second Jan’s awesome idea of incorporating other things that they enjoyed or that remind you of them. You could also consider less personal (but very traditional) general offering items such as incense, foods (especially Soul Cakes), salt, etc.
Wishing you all the best as your honor this memory this Samhain
Blessed be! 