"Are your T-shirts dirty?"
Translation:¿Tus camisetas están sucias?
Your answer means "you are dirty T-Shirts". Estás means you are.
So you need to specify that the t-shirts are "yours". So either tus camisetas (informal) or sus camisetas (formal or plural) and then you need to ask if they are dirty. Because the t-shirts are third person plural ("they") you need to use "están".
So all together that should be tus camisetas están sucias. I hope I didnt confuse you further and maybe could help you
You have to use estan rather than son, since "being dirty" is a (usually temporary) state rather than an inherent description/characteristic of the t-shirts.
Additionally, maybe your word order is wrong, but I'm not positive. The re-ordered version "¿Estan sucias sus camisetas?" was accepted for me, at least.
Spanishdict.com is a very good reference for this but you have to drill down into the definition of Ser and the definition of Estar. Both will have click for conjugations then it will also have a circle with a question mark inside it. Using all this I have finally figured out the "tree" that explains everything not just one part. https://www.spanishdict.com/conjugate/estar
"Tus" is possessive plural, but what's plural is not the possessor (tú). The possessed (camisetas) is what is plural.
Tu camiseta --> your shirt
Tus camisetas --> your shirts
In both, only one person is the owner of your shirts.
For "y'all", you would use ustedes. The possessive pronoun for that is su/sus. (I don't know much about vosotros so I won't risk leading anyone wrong.)
On mobile so not sure how old this question is. Hopefully this helps someone though.
Also, I'm not a native Spanish speaker so if anyone sees any errors in my explanation please correct me!