Even though also English is a part of the Indo-European language family, it stems from the Germanic Branch of the family tree. Both languages have the same origins
I'd agree with you up to a point, but the addition of Norse, Old French plus the residual Celtic makes English far more complicated than German itself. And the grammatical structure may have stemmed from a Germanic root, but it has become a great deal more flexible through incorporating elements from French.
Is there anything you actually know about, rather than just bullshit about in a pseud-y way?
1) There is no "residual Celtic" in English. There are a handful of words, nothing else. Similarities in grammatical structures between Celtic languages and English exist entirely because they are both Indo-European languages, and not because of any absorption of "residual Celtic" (by which I presume you meant Welsh or Gaelic, or whatever specific Celtic language you might think of).
2) Old Norse is also a Germanic language, and was so linguistically similar to Anglo Saxon that the integration, or cross-pollination of the languages resulted in a simplification of English grammar, rather than the reverse. Because Norse and AS vocab are similar enough to be mutually intelligible, while the major differences are in the inflected endings, the hybridised language evened out the declensions in the endings. That is, the grammar was SIMPLIFIED.
3) Norman French added hugely to the vocabulary of the language, and again resulted in a simplification of grammar. Middle English becomes, like Modern English, strongly dependent on word order rather than grammatical endings to convey meaning. So, in actuality, the language became LESS FLEXIBLE in terms of form, but far more flexible in terms of vocabulary.
Examples, expanded explanations and general resources can be provided on request.