I have requested this thread should be moved to the Lufthansa / M&M forum.

In the meantime you may want to check Lufthansa’s seat reservation info page.
https://www.lufthansa.com/de/en/sitzplatzreservierung

Whether you can select your seat in advance or during check-in depends on your fare class.

“If you are travelling on a European route on the Economy Light fare, you will be automatically allocated a seat at check-in. An alteration fee starting from EUR 29 will be charged if you would like to change this seat.”

So you may be able to find where you and your husband are seated during online check-in (23 hours before departure) and see if your sister can pay EUR 29 to move to a seat next to you (assuming one is available).