Slavic linguists stated proto-Slavic is a daughter branch in relation to proto-Baltic.

There are east (Lithuanian, Latvian, Latgalian) and west (Prussian, Yotvingian) languages. Some linguists state proto-Slavic separate from proto-Baltic (west and east) languages. Other linguists suggested  that proto-Slavic and western Baltic separate from proto-Baltic. Yotvingians and Prussians  spoke a similar language to Slavs in comparison to eastern Baltic (Lithuanian, Latvian, Latgalian) languages.

In northern Belarus and eastern Lithuania people consider each other cousins.  Leading Lithuanian scholar is stating Belarusians are cousins to  Lithuanians. http://www.delfi.lt/news/daily/lithuania/lietuviu-proteviai-skandinavai-o-pusbroliai-baltarusiai.d?id=20646059

An eastern Lithuanian woman speaks standard Lithuanian.   Does she look and sound as an eastern Slav to you? Start listening at 6:20.