You can only marry a step-sibling in the UK if the 2 parties first met as adults. If one or both was under 18 when they first met, then no.
Edit: I believe the same applies for step-parent/child.
Edit: I believe the same applies for step-parent/child.
"If one or both was under 18 when they first met"
The age of consent in the UK for most relationships is 16. It's raised to 18 in the case of many step- relationships.
The age at which they met is irrelevant; the age at the moment of the activity – marriage/sexual congress – is the relevant matter in law. Once both parties are older than 18 there is no legal impediment.
1. Romeo and Juliet meet when they are teens. They are not step-relatives. They develop a romantic relationship, which their parents disapprove of.
2. The maintain their relationship, which grows deeper with age, and plan to marry as soon as the are 18.
3. Before they turn 18 both of them lose a parent (maybe to divorce, maybe to death). Their remaining parents get married in order to make Romeo and Juliet step siblings and thus block them from marrying.