Like I said in the title, I am aware that Scots is a sister language of Modern English. I am also aware that Frisian diverged from west Germanic, making it and it’s modern variants sister languages, but is there a language, dead or alive, that diverged after Latin and the romantic languages influenced English(middle English)?
Edit: Made the question clearer.
Edit: There’s some confusion with the definition of dialect vs language. I would assume the definition in this case would be a language that diverged from English between the early late English period to the late middle English period.
According to this chart, the sister language to English is Frisian…
Conclusion: unless you precisely define what the term "sister language" means, there is no way to answer.