As she is not a relative by blood (or even, really, by marriage as a step-sister would be) then there almost certainly isn't a term for your relationship. Having said that the word cousin or the phrases distant cousin or sort of cousin can cover almost any family relationship.
The Oxford dictionary gives these definitions
cousin (also first cousin)
1 A child of one's uncle or aunt.
1.1 A person in one's wider extended family, to whom one is not closely related.
‘she's a distant cousin’
1.2 A thing related or analogous to another.
‘the new motorbikes are not proving as popular as their four-wheel cousins’
1.3usually cousins A person of a kindred race or nation.
‘our American cousins’
1.4historical A title formerly used by a sovereign in addressing another sovereign or a noble of their own country.
Also note that the relationship is not included in the Table of Kinship and Affinity from the 1662 Book of Common Prayer so people having that relationship can marry under Church of England rules.