The only way to find a link, if one exists, is to research each family, and see where it leads.

A quick look at the Sally Shelton baptised at the Mill St Independent, Bedford, on 14 Nov 1806, parents William and Frances (Lavender) shows that this couple also baptised a Susannah on 9 Nov 1811 in the same non-conformist church - perhaps this is the Susannah who was the witness at Sarah's marriage. Would take more research on Susannah to prove it.