People often do not understand other dialects of their own language.

In terms of individual words, languages will often take loan words from other languages, so for example Turkish has a lot of Persian words as the ruling class used to speak Persian. Or English is something like 1/3 loan words from French due to the ruling class speaking it. But if you go to pre Norman English (Anglo Saxon) it is much closer to early forms of Dutch (Friesian).

This is why when you have ancient forms of languages you can see more clearly the connections.

Also with Indo European the languages likely started to split about 6000 years ago when the various people migrated east towards India and Iran and west into Europe. So there is a huge amount of time for the languages to diverge. The similarities are in the shortest and oldest words, words for things like father, mother that kind of thing.