@annieeg have just measured all my windows (I was curious and wasn't sure!). Mine are all 4-4 1/2" that's 10-11cms. Some of my curtains I've made but others are bought and all the 'drops' are 54" or 137cm. Which is one of the standard lengths if you are buying ready made curtains.
It might be an idea to ask what is the drop from the top of her curtain track to the bottom of the window sill. Also maybe you could ask her to measure the actual window drop just so you know what you're working with.
Just another thing. I'm assuming the new track will be fitted in the same position, ie height as the old, but do the curtains 'hide' the track when they are closed or do they hang below with hooks that are visible like they would from a curtain pole? Hope this makes sense! If it's the latter the length will need to be adjusted.
With the 3" deep tape there are usually 3 tiers of slots for they curtain hooks. This will allow you to 'adjust' the length slightly if they will hide the track eventually. I usually use the top rung but it can depend on the track.
Is there a radiator under this window? Mine sit about 4" above the radiator but again it's a personal choice. Perhaps you could ask your sister where her other curtains finish in other rooms if they are sill length.
Sorry to be so long winded but we had a very good teacher, her daughter had a soft furnishings shop and she gave a talk on all the prep you should do before you start to measure for your curtains.
I'd err on the side of caution before cutting as you can't put it back.