Since you can't find an Amelia Simpson death registration between the December quarter of her marriage in 1867 and the December quarter of her sister's marriage in 1868, both in Westminster, I would first search on the premise that in the grief of her early death, her registration was incorrectly registered. With a more common surname like Simpson and in an area with an extremely large population this is obviously a risky hypothesis to try and prove. There is only one Simpson death registration in Westminster for that period
Ellen Simpson aged 21 in the December quarter of 1867. Not a great match because I believe your Amelia was born in 1849. However if she married under age she would have been likely to have given the more convenient age of 21 - saves on the paperwork.
Amelia's death of course could just have been left out of the central index. There was no system for checking that clerks were indexing everything correctly, as the returns came in from the local registration offices.
I'm surprised the second marriage took place in the same area, as the marriage was within the prohibited degrees. Not until the Deceased Wife's sister's Act of 1907 was such a marriage allowed (Deceased Brother's Widow marriage act was not until 1921). The Church of England did not come in line with these two marriage acts until the 1940s, so if the church had known it would not have allowed the marriage in church. Usually when such marriages took place the couple concerned married out of area by licence, so their relationship was not known and there was no chance of a local parishioner intervening if banns were called.
Regards
Valda
