1. I would call the initial person in a family tree 'the root' or 'root person'.

2. Sister, mother etc. are the names of the 'relationship' of an individual to the root person. Special subsets of related individuals include 'ancestors' (from whom the root person is descended) and 'descendants' (to whom the toot person is an ancestor). All individuals in a tree who can be linked to the root person are their relations or relatives (both words are used).

1. I would call the initial person in a family tree 'the root' or 'root person'.

2. Sister, mother etc. are the names of the 'relationship' of an individual to the root person. Special subsets of related individuals include 'ancestors' (from whom the root person is descended) and 'descendants' (to whom the root person is an ancestor). All individuals in a tree who can be linked to the root person are their relations or relatives (both words are used). How close the relationship, and whether direct or by marriage, depends on the exact path through the tree that links the root person and any single relative.

I would call the initial person in a family tree 'the root' or 'root person'.

1. I would call the initial person in a family tree 'the root' or 'root person'.

2. Sister, mother etc. are the names of the 'relationship' of an individual to the root person. Special subsets of related individuals include 'ancestors' (from whom the root person is descended) and 'descendants' (to whom the toot person is an ancestor). All individuals in a tree who can be linked to the root person are their relations or relatives (both words are used).