SOLVED. Chapter book about a group of siblings, possibly step-siblings, set around early 20th century. [s]

[Andria]

Your description made me think of The Penderwicks: A Summer Tale of Four Sisters, Two Rabbits, and a Very Interesting Boy by Jeanne Birdsall
https://www.goodreads.com/book/show/8...

[Holly]

I'm going to guess it might be an E. Nesbit book. Try looking at Five Children and It or The Story of the Treasure Seekers.

[Rachel]

Seconding the suggestion for Five Children and It. There's a part near the end where the siblings wish the baby would grow up, and he suddenly becomes a young man and walks off and leaves them, which sounds similar to Megan's description.

[Lobstergirl, au gratin]

Megan, did you check out these suggestions?

[Holly]

That's too bad. Let's see what else it might be.

You should take a look at:
Linnets and Valerians
The Saturdays
What Katy Did
The Story Girl
Five Little Peppers and How They Grew
Half Magic
Rainbow Valley
All-of-a-Kind Family
The Family from One End Street: And Some of Their Adventures

[Rainbowheart]

Does anyone know which book this turned out to be?

OP's account is deleted.