polly, we didn't have any contact whatsoever after the adoption, but we became reacquainted this past December, almost serendipitously.
I don't think there's a Spanish equivalent for half-niece or half-nephew. You would just say "sobrino" or "sobrina." Remember, in Hispanic culture, it's not unusual to have extended families. Bu if you insist, there's "sobrino/sobrina lejos," which literally translates as "distant nephew/niece."
And in case you're wondering, half-brother and half-sister is "medio hermano" and "medio hermana."
Salve, Regina, Mater misericordiae
Vita, dulcedo, et spes nostra
Salve, Salve Regina
Ad te clamamus exsules filii Eva
Ad te suspiramus, gementes et flentes
O clemens O pia