What is the plural form of your sisters school?

The plural form of the plural noun sisters is sisters'.

The English language can be very confusing when it comes to pluralization of possessive punctuation. Perhaps these examples will help you:

If you have more than one sister:"My sisters' school is..."

If you have one sister who goes to more than one school: "My sister's schools are..."

If you have more than one sister and they go to different schools: "My sisters' schools are..."

Please note that the difference is the placement of the apostrophe, not the pronunciation.