Final answer:
Alex originally had three sisters, and despite the death of one, he still has three sisters since familial relationships are permanent. Therefore, Alex has three sisters now.
Step-by-step explanation:
The question presents a scenario where Alex has three sisters, and mentions that one of them is deceased. This is primarily a question about understanding the permanence of relationships regardless of life status, which can fall into the category of logical reasoning within mathematical thinking rather than an arithmetic problem. Even though one of Alex's sisters has passed away, Alex still has three sisters. Death does not change the fact that Alex was born with three sisters, so the number of siblings does not decrease. The biological relationship remains.
In providing a final answer, it's important to recognize that the question may also touch upon understanding the meaning of family relationships in different contexts. This can often be confusing for students, as the concept of counting individuals can differ based on context-specific information; however, in this case, the familial bond is considered permanent. Alex has three sisters. The passing of a sibling does not change the number of siblings one has.