You should have posted this is in LH forum for better and faster advice.

It has been a while since I did OLCI on the LH website or app, but there might be a feature where you are able to select/add persons on separate PNR when you check-in. Never used it, but I assume that the idea is to seat the pax together like if they all had been in 1 PNR. If none/it fails you ask at the check-in counters if they can reseat you. Last resort is to ask onboard if someone is prepared to swap seats with your sister. Unfortunately, there is a high chance that you sister will not end up in a seat close to you.