Are the siblings of my foster mother considered my siblings, especially since they still cover themselves in front of me despite us all being in our thirties?

Your foster sisters are forbidden to you, based on the Almighty’s saying: "And your foster sisters." Similarly, their sisters from their father's second wife are forbidden to you, due to your shared suckling from the milk produced as a result of their father's marriage. This issue is known as "laban al-fahl" (milk of the male), and it establishes prohibition according to the most preponderant opinion among scholars.