Do you talk to her about your life?
The fact that you are busy with your children and your job, which takes up X hours per day, what time you get up in order to get DCs organised and you out to work, how little spare money you have, how you would be completely lost without your sister's help, money, money, money, time, time, time.
You need to be clear and direct with her - a mother of young children who is just about keeping afloat financially doesn't have the time or the money to spare for a hiking trip. On top of the cost of the trip itself there's the cost of childcare while you're away or the obligation on your sister to take care of the children.
Same goes for German language classes. You don't have the time because you have children to feed and put to bed in the evening. You don't have the money because you have child care to pay for and bills as well and you make.
These elements of your situation may not be obvious to her. I would consider it odd that a grown adult would genuinely not understand that there's a reason you have a job.
But it looks as if you are going to have to spell it out for her with absolutely beating about the bush or hinting.
If you value her friendship, ask her to go on a picnic in the park with you and the DCs, or hang out with you all at home. That way she'll see that there are demands on your time and energy.