I hope you can visit your seniors when you have time, such as the "Eleventh" and "May Day" holidays.
If you really don't have time, you can always call to express your concern. You can also send some small gifts.
For example: the special food of your city or some trinkets. (as long as you are willing to spend money)
There are also Chinese New Year, festivals, sending messages to express blessings and so on.
I'm sure your senior sister will be very happy as long as you get what you want.
In addition, don't be too eager to express your feelings, because she will be very happy if you owe her a favor, because
You should continue to get along well. So it's good to have a psychological bond with each other, which will make your have a better relationship.
If you are eager to express your gratitude, you will make your senior think that you don't want to associate with her anymore!