Is The dutiful relative pleases you also acceptable?
I couldn't try that as I was using tiles (I make too many typos, which annoy me, so I find tiles allow me to concentrate on translation!)
It definitely should be, as Duo translates "you like the bed" with "lectus tibi placet" with can also mean "the bed pleases you" or "the bed is pleasing to you". See: https://forum.duolingo.com/comment/34663094
"Pius" is pronounced "fius" in this audio - is because it comes after an s? Or like p and ph being interchangeable?
No. P should be pronounced as unaspirated, and PH should be aspirated. F is not the same as PH in classical Latin.
You like? It thought that this verb is in this sentence in the third person or am I wrong?
The literal translation is "the dutiful relative is pleasing to you", but it's often translated as "you like the dutiful relative". It's just like "ti piace" in Italian, or "te gusta" in Spanish.
Why not "cousin"? The word is used in a general way to mean "relative".
"Cousin" is not general, it is specific.
"Relative" is general.
Parece ser un martirio para los angloparlantes.