It's unlikely that there's anything wrong with the machine that will need a professional repair. The machine was finished with shellac. Any harsh cleaner will strip it. Very gently use sewing machine oil as others have advised.
The tension unit is the part that's on the extreme left of the machine and sticks out. It looks like it's missing parts. These tension units are easy to find online. There are a number of reputable vendors you can contact and you can certainly find them on the 'bay. These parts are easy to replace. It's like screwing in a lightbulb.
In their manuals Singer advised that when machines have been sitting for a while and are working "heavily" or don't turn over easily, that one should drip kerosene into the oiling holes to dissolve old oil and grease. Keep the kerosene off the black finish though. You can use kerosene to clean the inside of the machine and the mechanism underneath. 99% alcohol also works, but not as quickly. Don't use 70% as that is 30% water and can cause rust. Alcohol will also dissolve the shellac finish, so be extra careful with it. Kerosene is less likely to do damage to the shellac, but don't drip it on the finish, and if you do, wipe it off immediately.
When cleaning the machine, be on the look-out for thread jammed up in the mechanism under the plate where the bobbin fits. These are relatively delicate mechanisms and very apt to collect pieces of thread and fluff which can jam up the mechanism.
Otherwise, oil the machine using sewing machine oil only. Tri-Flow synthetic oil has a good reputation. Never use WD-40 or 3-in-1 oil. With a proper cleaning and sewing machine oil where needed this machine will still be sewing 100 years from now.
Your sister will need to clean and oil the machine if she's going to use it, so she needs to learn how to do that. It will take any reasonably intelligent person about 10 minutes to learn what she needs to know to keep this machine going for the next 100 years.
This is a model 15 machine. The manual for this machine is easily found on-line and it will show how to operate and maintain the machine. Singer started making these machines in the 1800's and continued to make them for something like 100 years in Scotland, the United States and Canada. They are still being made by Singer in India today. Google Singer India and you'll find this exact machine, in a treadle or with a hand-crank being sold by Singer today.
Singer made these by the 100's of thousands over the years. They are built the proverbial tank. Better probably. All parts are easily available, and if a person is at all mechanically inclined these machines can be repaired and kept in proper adjustment by a person with one or two screwdrivers.
Last edited by 1.41; 12-20-2021 at 08:54 PM.