Translation:The strong siblings are carrying the table.
It is nominative case. But the inflection pattern of the adjective depends on the word before it. Here it's a definite article, so you have to use the so called weak inflection pattern:
„Die starken Geschwister…“ – „The strong siblings…“
No article / determiner in front of it would require the strong inflection pattern:
„Starke Geschwister…“ – „Strong siblings…“
A rule of thumb, when the article already has the require inflection (die or -e in this case), then the adjective takes either -e or -en (weakly inflected).
For weak inflection, all plural cases take -en. (also all dative and genitive cases, as well as after "den"; all other cases take -e )
"Siblings/Geschwister" means "brothers and/or sisters" not explicitly "brothers AND sisters". So you could use Geschwister if two brothers, or two sisters, or three brothers and seven sisters, etc were carrying the table. It's very flexible. But if you literally say "brothers and sisters" that's only applicable to the situation where both genders are present.