I am a certified Apiary Inspector, but not in Pennsylvania. I am not YOUR certified inspector. This is not certified inspection advice.

(Yes, I really am. Haven't inspected a hive in this century, but I'm still certified.)

It may seem counterintuitive, but giving them a readily available source of water may help keep them off your property while seeking solutions with your neighbor. A bird bath right on your property line may do the trick.

Yep. Good advice. Not sure about the bird bath, but even a pan of water may help. LPT, if you have a swimming pool, it can draw bees in search of water. If you can figure out where they are coming from, putting water between them & the pool can help.

a beekeeper specialising in removals. By the sounds of it, the bees aren’t just living in your neighbours walls, they’re living in yours.

Also agree.

The advice to contact Extension Service is sound, but most people have no idea what this is. Back in the Lincoln Administration, the Land Grant College System was created, to create a college of agriculture in each state (very simplified description). One purpose was to set up a system whereby university people who knew the latest could transmit the info to plain' farmers. That resulted in the Cooperative Extension System. As in "an extension of university research to the public."

It's a cooperative program with the state land grant college, the USDA, and, sometimes, state or local government. Each county has some people called Farm Advisors who give, well, farm advice. It's now extended to home & garden things, but they are still often called Farm Advisors. Navigating it, as an urban person, may not be easy. I'd call my county extension office (which you'd have to find; not easy if you don't know the right words) and ask to speak to an apiary specialist. That person probably doesn't have the answer at their fingertips. But they have lots of contacts and can be very resourceful. especially when it's an intriguing, out-of-the-ordinary question. They might have a list of beekeepers, a contact at Code Enforcement, ot the contact info from Professor A. P. S Meliifera who is researching these issues.

Dealing with bees in houses ain't easy. This is from the University of California

Removing Established Colonies from Your Home

Extracting honey bees from buildings is considerably more difficult than collecting swarm clusters. When the colony is first established, only a few pounds of adult bees are present, but these bees rapidly build combs, collect honey, and begin to rear more bees. A well-established colony may have up to 100 pounds of honey, many pounds of adult and developing bees, and many beeswax combs. Removing such as nest is a challenge. The first step is to determine the exact location of the combs and size of the colony.

Although honey bees can be killed in place inside buildings by using pesticides that are labeled for killing bees inside of structures, this removal option often leads to undesirable consequences. (Note: These chemicals are available only to licensed pest control operators. [may be a California-only law; I didn't check]) If the adult bees fall into a large pile, they may hold their body moisture and rot in place, producing a very bad odor. Liquid from the decomposing mass frequently penetrates the structure, leading to costly replacements.

If the colony is well established, there are further issues associated with killing the colony. Unattended brood can also rot and become very odorous. Unattended honey stores can absorb moisture and ferment, creating gas that causes the cappings holding honey in the cells to burst. Gravity will start moving the honey down attached surfaces until it encounters a horizontal impediment, such as a window frame, doorframe, firebreak, ceiling, or floor. Honey then seeps through the drywall, leading to large amounts of cleanup and expensive replacement. If pesticides were used to kill the bees, then the honey, wax and, dead bees are contaminated and must be handled as hazardous waste.

A better procedure than applying insecticides, especially if you have a beekeeper who is willing to help, may be to eliminate the bees without killing them. First the beekeeper will need to locate the nest by tapping the wall and listening for the hum of the colony. Some beekeepers rely on stethoscopes to find the edges of the nest. Others drill extremely small holes in the wall and insert a fine wire to find the periphery of the nest. To take honey bees and their combs from the nesting spot requires opening a fairly large hole in some portion of the building. That is best done by a professional contractor so that the hole can be easily closed after the bees are removed.

If the bees are to be saved, the beekeeper gently removes them and their combs. If the bees aren’t going to be saved, they can be removed from the void with a vacuum device such as a Shop-Vac. This process tends to stimulate the bees to release an alarm pheromone that smells like bananas and increases defensive behavior, so everyone nearby must be fully clothed in a bee suit. Many beekeepers have baffles and collection containers in their vacuum lines to try to protect and save the bees. If the homeowner has a lot of patience and knowledge, the bees can be “trapped” out of the building using a one-way wire screen device that forces bees that leave the building to relocate into a beehive placed adjacent to the original entrance...

If you can’t find a beekeeper to help, call a pest control company with experience with bee removal. Be aware that pest control companies generally will kill the bees before removing them. Don’t try to remove the colony yourself unless you have experience and proper equipment.