One of the people we visited over Thanksgiving was my sister-in-law Becky who makes the most delicious carrot soup. We had a little luncheon at her home and, among other things, watched Marilyn (my niece) practice sitting up. Alice had not really gotten the hang of sitting up yet without assistance. But it seems to me that Marilyn is a very good role model for her cousin because Alice chose that afternoon to emulate Marilyn and sit up on her own for a ridiculously long time.
Note: This video might be more interesting for Grandparent and Great-Grandparent types as it is very monotonous.
I think you can even hear Becky deadpanning the word, "fascinating" and Bro-in-law Mike saying, "Blog entry!"