Joshua Engel has a good answer on this topic in a Quora reply to the question posed above:
It kinda depends on what you call a “language”. There are a lot of creoles and pidgins that are close enough to English as to be mutually intelligible. They’re descendants of English, rather than siblings.
For siblings, English is classed as a member of the West Germanic family. The family tree is usually given as something like this:
The nearest sibling is, a Germanic language. Both languages have evolved a very, very long way from the common root, English by French, and Frisian by Dutch. Frisian is actually a collection of dialects, and all told it’s really hard to see any family resemblance between modern Frisian and Old English, even if you speak Old English.
For comparison, here’s the first lines of the Lord’s Prayer in Frisian:
Us Heit, dy’t yn de himelen is, jins namme wurde hillige.
and old English:
Fæder ure þu þe eart on heofonum; Si þin nama gehalgod
and modern German:
Vater unser im Himmel, geheiligt werde dein Name;
At a casual inspection none of these look much like “Our Father who art in Heaven, hallowed be thy name.” The roots are actually all there: none of these words come to English via French. But to see it you’d really need to look into history rather than spelling: the Anglo-Saxon tribes that came to the island that became England also ended up in Frisia.
These words actually show more of the Norse Viking influence on spelling:
Faðer uor som ast i himlüm, halgað warðe þit nama.
Which became modern Norwegian:
Vår Far i himmelen! La navnet ditt helliges.
The Old Norse looks considerably more like the Old English than modern Norse does.
So the answer is usually given that Frisian is the closest, but it doesn’t mean you’re going to have an easy time learning Frisian. And in fact I found it far easier to learn French (and other Romance languages) from English than I did German, whose syntactic structure is now very different from English.