If I asked you to name two languages that people often say are similar, what languages would you name?
Maybe Spanish and Portuguese popped into your head. Maybe you thought of German and Dutch. Or maybe Hindi and Urdu. But what about English? Does it have a closely related sister language?
One group of people say that Scots is the closest language to English.
Scottish English?
Scots is not the same as Scottish English. It is a language that broke off from Northumbrian Middle English and developed in its own direction while standard English came to be based on southern English dialects. Now Scots, especially when its spoken in its most authentic form, sounds closely familiar but is not fully intelligible for most English speakers. It is written differently from English, with spelling that reflects its different phonology, it contains different words, and so on. There is no universal criteria for what constitutes a distinct language vs. a dialect, but Standard English and Scots are as different or more than Malay and Indonesian, Hindi and Urdu, Serbian and Croatian, maybe even Spanish and Portuguese (depending on the varieties of English and Scots being used).
But some people insist that Scots is simply a historic dialect of English, and is comparable to the English dialects of northern England. Does simply crossing the border into Scotland turn a dialect into a language? Does simply spelling words the way they sound turn your accent into a language? Does using regional “slang” mean that you speak a different language? These are all questions asked by people who think Scots is a dialect of English.
If, and I do mean if, Scots is not a distinct language from English, then what is English’s closest relative? Technically, the answer is Frisian, or the Frisian languages.
Frisian
Indeed, you can easily see the resemblance between English and Frisian in this classic pair of sentences.
Frisian: Bûter, brea, en griene tsiis is goed Ingelsk en goed Frysk.
English: Butter, bread, and green cheese is good English and good Fries.
But in the comments written in response to the video, a fair number of people have written that Frisian resembles Dutch and German more than it resembles English. And others have written that English seems to have more in common with creoles like Tok Pisin and Jamaican Patois than it does with Frisian. In terms of mutual intelligibility, that may be the case. What makes English and Frisian sister languages is their genetic relationship, or in other words, their common origin.
Frisian belongs to the same Anglo-Frisian sub-branch of West Germanic languages as English. Anglo-Frisian is further divided into the Anglic languages (English and Scots) and the Frisian languages. 
West Germanic languages (including Dutch, German, Afrikaans, and Yiddish in addition to English and Frisian) share many cognates, but the Anglo-Frisian sub-branch has some unique phonetic features.
Palatalization of g
Have a look at these cognate words:
English day, Frisian dei
English rain, Frisian rein.
The Dutch cognates are dag and regen. And the German cognates are Tag and Regen.
Do you see how English and Frisian are different from Dutch and German? The letter “g” became palatalized (and came to sound like English “y”, or /j/ in the IPA) before or after certain vowels. Those examples show the change after the vowel, while this next example shows the change before the vowel:
English yesterday
Frisian juster (“j” is pronounced like the English “y”, IPA /j/)
Dutch gisteren
German gestern
Palatalization of /k/
Another example is the palatalization of k, resulting in an affricate.
English cheese
Frisian tsiis
Dutch kaas
German Käse
English church
Frisian tsjerke
Dutch kerk
German Kirche
Loss of “n” before voiceless fricative
Another example is the loss of “n” before voiceless fricative f, th, or s.
English us
Frisian ús
Dutch ons
German uns
English goose
Frisian goes
Dutch gans
German Gans
And there are several more phonological changes common to English and Frisian that set them apart from other West Germanic languages.
Of course, phonology isn’t everything, and when it comes to syntax, Frisian often shares more in common with Dutch and German than it does with English, in part due to the influence of Dutch and German over the centuries. It is also closer to Dutch and German in terms of vocabulary, since English absorbed a large amount of its current vocabulary from Norman French during Norman rule in England, conditions which didn’t affect Frisian or other West Germanic languages. That’s one reason why a creole like Tok Pisin or Jamaican Patois is probably more mutually intelligible with English than Frisian is. They received most of their vocabulary from English after English vocabulary had been greatly influenced by French.
But when linguists talk about which languages are closely related, they are specifically talking about the origins of those languages and disregarding all of the additional influences they have undergone along the way. By that criteria, there is no closer relative to Anglic (English and Scots) than Frisian.
