A small girl visits (?) her grandmother or aunt who is a good witch albeit not a very powerful one. Somehow an evil witch who keeps as a pet an octopus named Otto gets both in danger. In the end the good witch prevails.
Stump the Bookseller
New format, same book stumper solving.
Could it be this previously solved stumper The Letter, the Witch and the Ring by John Bellairs?
http://w1.loganberrybooks.com/stumpthebookseller/?p=3137#comments
How about Nightshade by Rachel Weininger?
A magical black cat helps two young girls unravel a spell that has been cast over their Aunt Maude who is a “white” witch.
https://www.goodreads.com/book/show/2020606.Nightshade?from_search=true&search_version=service
Last guess: Timothy and Two Witches by Margaret Storey.
When Timothy goes to stay with Melinda, his life takes a magical turn because she is a white witch with doors that open by themselves, dragons in the garden, summer when it is winter everywhere else, but most fortunately, he finds a new friend, Ellen, to explore with!
https://www.goodreads.com/book/show/1558562.Timothy_and_the_Two_Witches?from_search=true&search_version=service
Nope, unfortunately that’s not it. The key to it seems to be the octopus Otto, companion tithe evil witch. Thanks so much for trying!!
Here’s another guess: Margaret Elliot, Witch’s Gold, 1979. This book is about a witch called Bathsheba, who everyone calls Bessie, she has her two grandchildren come to stay, called Hugh and Sophie, who are also witches. Her broomstick is disguised as an ash sapling. She is a good witch, but a bumbling one, and all her pots of spells always get mixed up and she picks the wrong one. Her and her grandchildren are in a race against time to prevent the Black witch, Marcia, from carrying out her plan to steal an ancient manuscript with the formula of the Philosopher’s Stone, which turns metal into gold. The white council already have a plan to stop her, but Bessie and the kids try to help and everything goes pear shaped
Just a guess – “Which Witch?” by Eva Ibbotson? A bunch of black magic witches are vying to become the wife of a warlock, and one of them has an octopus as her familiar. There is one “white” witch who ends up winning the contest – Belladonna.