Are Twilightsworn considered Lightsworn?
Yes they are also considered Lightsworn cards.

The key part is to notice that "Twighlightsworn" also contains the word "Lightsworn" on its name, so they are also considered part of the Lightsworn archetype.

Another case are the Tellarknight achetype, which include cards with "Satellarknight" or "Stellarknight" on their names. Example cards that belong to this archetype are Tellarknight Ptolemaeus, Satellarknigth Vega, and Stellarknight Triverr.

Other strange cases are cards like Darktellarknight Batlamyus (also Tellarknight archetype), and Satellarknight Zefrathuban (which is both Tellarknight and Zefra archetypes).



In case this was not convincing, there is another reference from the Yugioh Wikia that states:


  "Twilightsworn", known as "Twilightlord" (トワイライトロード Towairaitorōdo) in the OCG, is a series of DARK monsters introduced in Code of the Duelist, and a sub-series of the "Lightsworn" archetype. 


Furthermore, if we compare the Japanese names of both we can see they share a same root which enables them to belong to the same archetype. 

Lightsworn or "Lightlord" in the OCG is spelled "ライトロード Raitorōdo", and Twilightsworn or "Twilightlord" is spelled "トワイライトロード Towairaitorōdo".

This is similar to the case of the Archfiend archetype, which due to translation from Japanese, includes now all cards that in their OCG names include the "Daemon" character (デーモン). 

This means that cards like Summoned Skull, which OCG name is デーモンの召喚 (Summoned Daemon, translated), are part of the Archfiend archetype.