Are these two usages correct?
My sister and I have last texted each other at 4 o'clock, I haven't heard from her since then.
My sister and I have last seen each other in the past year. We haven't seen each other ever since.
Do I use "since then" for shorter period of times and "ever since" for longer periods?
If they're not correct, would anybody please explain to me why they aren't?
I think you will find the usage of ever since varies with location and dialect. All I can do is tell you how I, speaking midwestern AmE, use ever since and why I use it that way. In most cases you can just use the word since.
The issue with the word since is that it can establish motive or it can establish a time frame.
This establishes motive -
Since Bob was taller than Oscar he placed the star on their holiday tree.
This establishes a time frame -
Bob placed the star on the tree and has not seen Oscar since [he placed the star on the tree].
But what if I want to begin my sentence with the word since. An issue of understanding arises when I start my sentence, which is intended to establish a time frame, with the word since. When the first word of a sentence is since I immediately understand it to mean motive.
Since Bob placed the star on the tree he has not seen Oscar.
The previous sentence cannot be understood out of context. Did Oscar disappear because he doesn't like stars on trees or has Oscar been gone since the star was put on the tree? I first think Oscar doesn't like stars but then when I think about it again I am merely confused.
To prevent that confusion and establish upfront that I am using since to establish a time frame, I include the word ever (a time word).
Ever since Bob placed the star on the tree he has not seen Oscar.
No confusion.
In answer to your question, Ever since is not only used to designate the magnitude of a timeframe but is also used to introduce or clarify since as a time measurement. Your sentence reads much better using since then because I already understand that it represents a time frame and the negated ever since sounds out of place at the end of your sentence.
You can use ever since at the end of a sentence to establish magnitude but not in the way you did it. You cannot negate ever since. Not ever since is awkward, confusing, and not commonly used. The negation of ever is never but we do not say never since. We would use never before the verb and end with since.
It is used like this -
He was elected governor in 1990 and has remained governor ever since.
or
He lost the governorship twelve years ago and has never been elected to any office since.
While the meaning of ever since and since then appears to be the same in the examples you cited, it can be different in different contexts.
Usually, since then is used when you are mentioning a point in time and are talking about something that has happened between that point (in time in past) and now. Example: "Last year, I replaced meat with vegetables in my diet and I have lost 10 pounds since then."
Ever since is commonly used when you are referring to a point in time in past when a particular action began and you are drawing a co-relation with that point in time. Example: "Ever since I watched an amazing documentary on science while waiting at the airport, I cannot stop searching and watching documentaries!"
Do you see the difference? When using since then, you are referring to thing between a point in past and now (I texted my sister at 4 and I have not heard from her since then) BUT when you are using ever since, you are referring to an action that began from a moment in past and it's still ongoing (I went to a science conference last year and I have been reading about science ever since).
I hope it's helpful.
With regard to ever since and since then both the sentences are correct. But the correct form of the sentences is --
My sister and I last texted each other at 4 o'clock, I haven't heard from her since then.
My sister and I last saw each other in the past year. We haven't seen each other ever since.
Ever since is stronger than since then
