We live in a 100-year-old house. The sewage and wastwater from the north side of our house drains into a pipe that is about three feet inside our neighbor's property line between our houses. This pipe connects to the city pipe in the alley. The original pipe is clay and crumbling. We need to replace about six feet of the old pipe that runs under her property. These pipes have been there more than 50 years; longer than we or she have owned our properties. Our neighbor will fight our access to her property. What rights do we have in this case?
Thank you.

There should be a easement to this on the deeds.

If there is no easement, it is very possible you will have to put a new line in on your property.

This may turn out to be a joint pipe in older areas it is not uncommom for two houses to use one main line to the sewer that is located between the two homes. ( I lived in one of these and it was a nightmare)

You need to search the deed records going back 100 years and see if a easment was ever made.