Considering "Sebastian" as a Slavic-sounding name (though its origins are more complex, encompassing multiple influences), here are some sibling name suggestions, categorized for different preferences:

For Siblings with a Similar Slavic Feel:

• Masculine: Dmitri, Boris, Igor, Pavel, Vladamir, Nikolai, Roman, Alexei
• Feminine: Irina, Natalia, Svetlana, Olga, Yelena, Anastasia, Milena, Zoya

For Siblings with a Softer, More Modern Feel:

• Masculine: Elias, Julian, Caspian, Leo, Ethan, Felix
• Feminine: Willow, Hazel, Clara, Eleanor, Audrey, Isla, Violet

For Siblings with a Strong, Classic Feel:

• Masculine: Alexander, Nicholas, Theodore, Arthur, Edward
• Feminine: Victoria, Catherine, Elizabeth, Margaret, Eleanor

For Siblings with a Unique, Less Common Feel:

• Masculine: Lysander, Silas, Rhys, Arlo, Jasper
• Feminine: Genevieve, Theodora, Juniper, Wren, Indigo

Considerations when choosing:

• Sound: Do the names flow well together when said aloud? Do they have similar or contrasting sounds that create a pleasing effect?
• Initials: Do the initials create an unintended acronym or word?
• Length: Do you prefer names of similar length or a mix of short and long names?
• Meaning: While not strictly Slavic, researching the meanings of names can add personal significance.

This list provides a starting point. The best sibling names will depend on your personal preferences and the overall aesthetic you desire for your children's names. Consider experimenting with different combinations from these lists, or using them as inspiration to find other names that resonate with you.