Complex sentence: The Brazilian government refused to recognize Maria Amélia as a member of Brazil's Imperial House because she was foreign-born, but when her elder half-brother Pedro II was declared of age in 1840 he successfully intervened on her behalf.
Split into simpler sentences: The Brazilian government refused to recognize Maria Amélia as a member of Brazil's Imperial House because she was foreign-born. However, when her elder half-brother Pedro II was declared of age in 1840 he successfully intervened on her behalf.