I suspect the first Evelyn died in 1918 as according to your headstone find, because without a doubt there is an 1913 birth record for her as the daughter of Stephen and Alice/Alicia.
Likewise, the death record is very definite about her birth in 1921 and I also believe this is true; I suspect she was named after her sibling that had died, that happened a lot.
Thing is, the records for 1921 are sealed; all Nova Scotia will present online is birth records over 100 years old, her birth doesn't qualify. I suggest you get in touch with the NS Vital Records and apply for a copy of her birth certificate as a descendant.
The marriage limit online is 75 years, so I suspect that is why this can't be found online either. The limitation for death records is 50 years, that's why her death at such a young age is available to you.
I would also say the marriage Edward outlined below is accurate, but I haven't seen the record about father Stephen dying in 1915..although that would blow my theory if he did!
I will add that I found they had another daughter Ruth Mary on July 8 1915. That perhaps gives you another avenue to research for more information and a picture.
The other thing I will add is that I see there were 4 Stephen's born in Bay Roberts in 1885!! I think you will need to find his death certificate for his parent's names if you are to get back any further!
Regards
Carol