POD is biologically quite implausible (Henry VI having a twin and still being more or less the same person) but I'm curious about political consequences, so let's go forward with that change.
What would be Henry's sister's (Mary?) fate and how would her existence affect situation of House of Lancaster? As royal daughter *Mary* is too high born for domestic match, OTOH pool of available foreign royals is quite limited-only royal candidates of right age in Western Europe are Dauphin Louis (future Louis XI)-but due to conflict with France he is unlikely, Henry IV of Castile (also unlikely) and Charles of Navarre, Prince of Viana.
What would be Henry's sister's (Mary?) fate and how would her existence affect situation of House of Lancaster? As royal daughter *Mary* is too high born for domestic match, OTOH pool of available foreign royals is quite limited-only royal candidates of right age in Western Europe are Dauphin Louis (future Louis XI)-but due to conflict with France he is unlikely, Henry IV of Castile (also unlikely) and Charles of Navarre, Prince of Viana.