Where is my tax nexus and how would I charge taxes and be taxed?

[kz20]

Hi, 

I'm a US citizen currently traveling in Asia. I'm opening a dropshipping Shopify store using my sister's address in the US.

For dropshipping I have China based suppliers for now and I'm mainly selling to US customers. I also plan to work with Printful, which would mean that I'll have a US based supplier for the POD products. In addition, I might expend my customer base to the rest of the world later. 

So my questions are:

1) How should I go about configuring my tax info?

2) If and how should I charge taxes for my products?

3) Would my "tax nexus" be considered in the US (at my sister's address)? Or would it be the country that I'm currently staying in?

Thank you.