Well, strictly speaking English has a closer kinship with German as they're both West Germanic languages, whereas the Scandinavian ones are North Germanic. That said, English and German have gone along very different paths of evolution for the last 1000 years. German has been quite conservative, preserving the four-case and three-gender system for nouns and still conjugating verbs depending on person and number. All of those features have been lost in modern English, which in turn has a quite peculiar vocabulary due to many centuries of influences from Norman French, Danish and British imperialism, to name a few.

Of all the languages to have influenced English, the French influence has been the heaviest. Swedish, on the other hand, has borrowed incredibly many words from Low German.

dem is them yes.

To be able to sing, you sort of have to be an animate thing, a human even. And most animate things are en. One exception would be ”barn” so if there are two children you could say that ”ett av dem sjunger” but otherwise it’s almost always en.

What Lundgren8 said applies to all pronouns in Swedish, for example, "nobody" is "ingen", but "nothing" is "inget"; "somebody" is "någon", but "something" is "något"; etc. For example, if you wanted to ask if there is someone in the room able to speak English, you would ask: "Finns det någon som talar engelska?"

As long as the subject is "one of them", the sentence follows the V2 rule.

I don't think from would work in English either – 'en från dem' sounds like it's someone they sent.
We don't use från for the meaning 'part of a group' in Swedish. Instead, it is most typically used to refer to a 'source', as in something comes from someone or from somewhere.