cant do a student id check in person, but need the student discount.

Student Discount programmes from Apple, where available, require that eligible students are enrolled in a recognised higher education programme within the country in which the qualifying purchase is made. Apple does not offer cross-border sales, and as such, you cannot purchase devices in one country and have them shipped to another.

Unless your brother-in-law is himself an eligible student in his own country/region - and can make a qualifying purchase on your behalf - you will not be able to benefit from educational discounts.

As you have already determined, in most countries, the key to accessing Apple’s Educational Discount programme is being registered with UNiDAYS:

Education Pricing and Student Discounts - Education - Apple

https://www.myunidays.com

If you are a student in country A attending a qualifying institution in country A,

you do not qualify for a student discount in another country. The Apple education

discount program varies by location and is not the same across all countries.

tldr; No.

In the US, this policy addresses the education discount applies:

Payment, Financing, Refunds and VAT - Shopping Help - Education - Apple

which includes:

 Please note that only orders placed on your country’s or region’s Apple Education site are eligible for the special education pricing.

I wish we could help more but you are not addressing Apple here, but rather other end users. Apple sales and promotion policies are beyond the scope of these user tech-help forums. If you need more clarification than in the link above, you will need for contact Apple, not other users.