Have a look at
Mittnik 2018, especially the supplementary information Figures 4 and 6a.
You should see a decrease of HG ancestry and increase of EEF ancestry.
I’d say it’s a result of people gradually moving across milllennia in several contexts:
- the Baltic language is Indo-European, with an origin in the southern forest-steppe;
- toponyms indicate that much of the central Russia west of Urals used to be Baltic, there may have been population movements towards the current Baltic areas;
- Lithuania shared a king with Poland during 1600s ans 1700s, that country extended almost to the Black Sea, covering most of today’s Belarus and Ukraine;
- the high percentage of Russians in Latvia mentioned by MOESAN that are possibly counted as “Latvians”, though Russians themselves have a variety of ancestries from Fenno-Ugric to Steppe, most of that migration is post-WWII and I believe the modern populations discount such recent migrants.
There was never much mixing with the Germans though, and Germans never moved to Lithuania either. Virtually all the Germans left 1918-1941.