Is my sister's son considered my son if I breastfed him a satisfying suckle and two unsatisfying suckles, and is suckling five times with a full mouth considered رضاعة (suckling/nursing that establishes a relationship of mahramiyyah)?

The suckling that establishes a prohibitive relationship is five or more satisfying sucklings. The reference for determining a suckling is custom. So, if the child suckles and then stops suckling by his own choice, that constitutes one suckling. If he returns, it is another. However, if he stops due to shortness of breath, or to move from one breast, or due to something distracting him, then immediately returns, there are two views: The first is that each instance is a separate suckling, and the second is that all of it constitutes a single suckling. It is reported in Sahih Muslim on the authority of Aisha, may Allah be pleased with her, that the prohibition is not established except by five known sucklings. It is also said that both little and much suckling establish prohibition, and this is the view of Malik, Al-Awza'i, and the jurists of independent reasoning.