My assumed premise:
Indo-European language classification is broad. We can always find two languages of this family which are grammatically so different, and also the languages grammatically similar.
But most of the study of this language family is based on comparative vocabulary and its cognates.
Most of the south Indian Dravidian languages have many Sanskrit words in them from times immemorial(it's not clear). For example Telugu vocabulary is mostly Sanskrit words that a Telugu person cannot(might not) give you a pure Telugu equivalent(Dravidian equivalent? of most of the words. It also attained morphological features and grammar features like Sandhi. Strangely enough now the trend is English words.
But it still is not classified under(not even in a partial sense) Sanskrit. The reason given is the language is not its words but its grammar. But the Indo European languages are classified mostly due to similarity in their words.
I understand this may be due to the fact that the Sanskrit words in Telugu, no matter when they might have been added, are loan words.
In this respect, I'm wondering what's the clear line between using grammar vs words in classifying world languages. Is it consistent across all languages? Or some language families have different peculiarities for their languages that made them under same language family?
For example for Dravidian languages, is it their grammar?
And also could it be possible that a language got influenced by other languages so much in many respects that it got classified in different language family than it should be? (Of course this might be ambiguous)