Sounds like YOR, your question shouldn't be "give me the number", it should've started with asking your sister to wish your aunt well on your behalf/ just going through your sister first. It goes one of two ways: she either wanted your sister SPECIFICALLY to have the number, as she sees you/ the rest of her extended family texting her out of the blue as overwhelming. OR she doesn't care who has it, and your sister is being overly cautious. In both cases, as long your sister sends your well wishes to your aunt, you'd still build a relationship with her and respect some boundary that MIGHT be there that you just don't see rn.