Firstly, the core of the matter is consent. Private parts are that: private. At such a young age of one, the generally acceptable person to touch their private area is their primary caretaker, and even then such touching needs to be done sparingly in a non imposing manner for a specific purpose.
At the time, her sister was one and generally we regard children that young too young to grant consent. Again, the only valid consent is affirmative consent. Just because her sister didn’t resist Dunham’s probing, doesn’t mean Lena’s actions were “alright”. We must also think about the implications now: what is Dunham’s sister feeling that this anecdote has been shared? Perhaps she sees it as a violation and must now deal with the psychological fallout of it as an adult.
Even if we accept this as simply “innocent child behaviour”, reportedly this 7-year-old-Lena, 1-year-old-sibling story is one of many similar shared anecdotes in the book. This means this kind of probing of her sister was frequent… Frequent enough for her one year old sibling in this particular anecdote to anticipate that Dunham would look into her vagina in order for her “prank” to be “successful”.
If stories of this nature are often told in this book, it means that Dunham is older in other instances of a similar nature and we expect older children to understand to keep their hands to themselves. The “innocent child behaviour” argument starts to slip…
Also, Lena is a writer as an occupation. She considers language and words. She was (is) an adult when writing her book. I can’t accept that she didn’t consider that her arguably innocent actions of youth would be okay to publicise now.