Got under the SSEi this AM. The lines are 3/16" . The two lines running inside the frame rail on the driver'* side run to the rear brakes. Just past the tight bend you mentioned behind the driver'* door, each line connects to its own proportioning valve. If yours are like mine, these valves are steel and rusted. Get a wire brush and some PB (Power Blaster) penetrating oil working on the fitting that threads into that proportioning valve. Do this today, and spray it a couple more times before you try to undo the line. Also, you might want to call the dealer for a price and availability on the valve, just in case. There appears to be no other fitting before all the lines connect to the ABS pump (?) way up front.
Here'* what I would suggest. Assemble the following: Double flaring tool with 3/16" dies. Cordless drill. 1/8" drill bit. Stubby tubing cutter. 6" flat file. Soft vinyl, rubber, or other plug material about 9/64" OD. "Practice" piece of 3/16" brake line. Use the practice piece to learn how to do the double flare, how to ream the ID of the pipe, and chamfer the OD. Find or make a plug that you can quickly and easily slide into the reamed pipe with a small tool like a nail or 1/8" drift punch. Actually practice doing this. Buy a replacement pre-fitted line about 24" and a coupler. Keeping the nut on the flared end, bend the line so that it matches the existing line that fits into the proportioning valve. Top up the fluid in the master cylinder reservoir. Tease the old line out of the plastic clips, put some nitrile gloves on, put a pan under, and use the close quarters tubing cutter to cut the line in a convenient place under the driver'* door. Quickly, with the cordless drill and an 1/8" bit, ream the line as the fluid is running out, then shove a soft plastic or rubber plug up into the line to stop the flow of brake fluid, like you practiced. With the line plugged, you can now take take your time to slip on a nut, and do a nice double flare on the line on the car. Disconnect the old line from the proportioning valve. (This may turn into a replacement of the poroportioning valve and another custom-fit flare on the downstream side of the proportioning valve.) Get the new line and coupler attached on the downstream side and ready to connect to the plugged end. Cover the end of the plugged line loosely with a rag and have a helper apply the brakes. The pressure will force the plug out. Make sure the plug comes out and that you have all of it. Connect the new lines together. Check for leaks; bleed the brakes.
If you find you cannot undo the fittings from the proportioning valve on either side, cut the line on either side of the valve and remove it from the car. With the lines cut flush aginst their rusted fittings and with the proportioning valve out of the car, you should be able to get box wrenches on the parts and separate them if possible. It may not be a good plan to put the valve in a vise to hold it. You want it to work when you are done.