Learn Polish: Bliźniacy są do siebie bardzo podobni.

Polish has two plural forms for nouns: the masculine personal and the non-masculine personal. Bliźniacy is the masculine personal form, used when referring to people where at least one is male. You might hear bliźniaki, but it is generally used for animal twins, very young children, or in highly informal speech. If they were twin sisters, the word would be bliźniaczki.

What does do siebie mean here? I thought siebie meant oneself.

You are right that siebie often means oneself, but in Polish, it is also used to mean each other or one another when talking about multiple people. Combined with the preposition do, the phrase do siebie translates to to each other in this context.

Why does the adjective podobni end in -i instead of -e?

The adjective must agree with the masculine personal subject bliźniacy. The basic dictionary form is podobny. For masculine personal plurals, adjectives usually change their ending to -i or -y, often softening the final consonant. Here, the n changes to ni, making it podobni. If the subject were non-masculine personal (like bliźniaczki), the adjective would be podobne.

What grammatical case follows the phrase podobny do?