Is my cousin (my aunt's son) considered my brother through rada'a (nursing), and consequently, are my sisters considered his sisters, based on my brief nursing from my aunt when I was young?

The suckling that establishes a prohibitive relationship is that which occurs within the first two years of life and consists of five full sucklings. The prohibition is not established when there is doubt about the number of sucklings, just as the testimony of a single woman regarding suckling is not accepted by the majority of scholars.

It is safer and more absolving for one's conscience to treat someone whose suckling is doubtful as a mahram by refraining from marrying them. However, they should be treated as a non-mahram (ajnabi) by not allowing the unveiling of the hijab in front of them, or being in seclusion with them, or shaking their hand; this is because the suckling is not definitively established. This falls under the category of caution, acting upon the hadith: "And veil yourself from him, O Sawdah." As for the other sisters, they are ajnabiyat to him, and it is permissible for them to marry him.