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Hi Ina,
You may well have found your answers by now, but just in case not....
Charlotte Augustus Marshall & Elizabeth Sexton (nee Marshall) were sisters

First name(s) Charlotte Augustus Last name Marshall Birth year 1823 Baptism year 1823 Baptism date 20 Jul 1823 Parish Ipswich, St Margaret Place Ipswich Relationship Daughter Father's first name(s) Edmund Mothers first name(s) Eliz Father's occupation Butcher Residence Ipswich County Suffolk Country England Source Suffolk Baptism Index (Part 3) Record set Suffolk Baptism Index 1538-1911

You do have the right marriage for their parents Edmund and Elizabeth

First name(s) Edmund   Last name Marshall
Marriage date 03 Nov 1794
Marriage place  Nettlestead
Spouse's first name(s) Elizabeth   Spouse's last name Rivers
Groom's parish Ipswich St Margaret County Suffolk
Record set Suffolk Marriage Index

In fact one of Edmund & Elizabeth's children was ;
Edmund Rivers Marshall b.17 Aug 1814 ; bapt St Margarets, Ipswich 14 Nov 1814

You will find Edmund Snr grave marker at this site;
https://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GSln=marshall&GSiman=1&GScid=2306492&GRid=155020724&

Hope this helps. I have a little more if you would like it.