Question:
Answer:
Once a child is adopted into a family, canonically the Church considers that child the same as any natural child from the marriage:
Those who are related in the direct line or in the second degree of the collateral line by a legal relationship arising from adoption cannot contract marriage together validly (can. 1094).
This prohibition exists for civil reasons (some civil laws may forbid such a marriage) and for reasons of general psychology. Two children raised as siblings who want to marry each other might be cause for concern about their stability.
However, this is an ecclesiastical law and as such can be dispensed by the local bishop. A situation in which someone was adopted as an older teen and the other child was away at college during the time the adopted teen lived in the house would be a situation where the bishop might dispense from the impediment.