Is marriage to a paternal cousin permissible or forbidden if the two of them were breastfed together? And does seeing marriage to her in a dream after performing Salatul-Istikhara mean that this marriage is permissible?

Suckling prohibits what childbirth prohibits, and marriage to a foster sister is forbidden. The jurists have agreed that five sucklings or more establish prohibition, but they differed on less than that. The majority held that both little and much suckling establish prohibition, even if it is a single suckle. As for the Shafi'is and Hanbalis, in their sound opinion, they believe that less than five sucklings does not affect prohibition, citing the Hadith of Aisha, may Allah be pleased with her, regarding the abrogation of "ten sucklings" by "five known sucklings." This latter opinion is the preferred one. Therefore, the questioner should fear Allah if he knows he was suckled five times, for she is his sister, and a dream has no bearing on this. When in doubt about the number of sucklings, the default is their absence, so the woman is not forbidden then; however, refraining from marriage is more prudent due to the difference of opinion on the matter.