Use of apostrophe

[KingsEnglish]

Hello everyone,
I would like a teacher to identify my grammar. I am getting older, and have muddied the waters of use of apostrophe to 'claim possession '. (Also placement of period, in or out?
My memory was that to show possession, a sentence would read
'My sisters' dog ran away'
Is that improper?
Please explain in detail.
Thank You,
N

 

[bhaisahab]

Hello everyone,
I would like a teacher to identify my grammar. I am getting older, and have muddied the waters of use of apostrophe to 'claim possession '. (Also placement of period, in or out?
My memory was that to show possession, a sentence would read
'My sisters' dog ran away'
Is that improper?
Please explain in detail.
Thank You,
N

If you have two (or more) sisters and they collectively own a dog, your sentence is correct.
If you have only one sister (or only one has a dog), "My sister's dog ran away" would be correct.