Spying from the outside seems like the logical and safest bet. You can still listen in, but much less chance of your sister spotting you and you having to face her retribution.
You consider your options. You could head up in the attic and listen in from above, there is a lot of old junk you’d have to to shuffle through and it would very dusty. It would also be hard to get up there without your sister noticing.
You could also listen in from underneath. Which you believe would be your mother’s office. Neither of you are really suppose to be in there without permission but if you were to stand up you might be able to hear some stuff through the roof if you listen closely.
Then off your you could listen in from this floor,
Next to her room is your parents room on one side and a broom closet on the other. Your parents master bedroom is also somewhere you shouldn’t really be, but your parents are out for the night and your sister will be busy, so maybe.
The broom closet is just full of cleaning equipment. Not really anything interesting but you might be able to get a good listen in from inside.
And then there is the tree house out back, if you camped out there and crawled out on a branch you might be able to see inside, best you the best view of what’s going on out of all these options. However hearing would be more difficult. Not to mention it will be cold and possibly dangerous out there.
You could always just ease drop from the door. But it’s more likely they’ll notice.
After your considerations you decide on your plan.