Same question as here but for Eastern Orthodox. I don't see how a married couple's becoming step-siblings is any different from step-siblings' become a married couple.
Suppose Alice and Bob are widows (or single adoptive parents or unmarried biological parents or whatever) and their respective children are Charlie and Dalia. Suppose all 4 are not married (or were never married or whatever, depends if widow or not) and none of them have ever divorced or annulled.
If Alice and Bob marry, then (I think...) Charlie and Dalia cannot marry. But if Charlie and Dalia marry, then can Alice and Bob marry afterwards?