This might be a wild guess, 六弟 means the sixth brother, but these shelters are not for J-6, since they are all retired. So maybe they are fo J-16D? since the pronunciation for 弟 in chinese is D, and there is a way of calling someone with close relation by their last word of the first name.
here's the translation:
"sixth brother's new home has 20 rooms."
and here's the pinyin per google translate.
"Liù dì de xīnjiā yǒu 20 gè fángjiān."
To be honest it can mean absolutely anything. Aside from what yuxiao had guessed, here's my two cents. Maybe it's a hint that a new version of something is coming and the shelter is for those? or maybe there is some unit getting relocated and those new shelters are for their planes?