How to use credit from Apple gift cards towards buying an Apple product

1.

See -> If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

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Code must be redeemed in a different country or region

If you see a message that says the code must be redeemed in another storefront, the card was probably purchased in another country or region. You can't redeem Apple Gift Cards or App Store & iTunes Gift Cards outside of the country or region of purchase. For example, if you bought an App Store & iTunes Gift Card in France, you can't redeem it in the App Store in the United States.

2.

See -> Check your Apple Account balance -> Check your Apple Account balance – Apple Support (UK)

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If you have an Apple Account balance that you can’t use

👉 You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards or unused Apple ID balance for cash, except as required by law. If your jurisdiction allows the refund of redeemed gift balances, you can contact Apple Support to request a refund. 

When you contact Apple, include the following information:

•           Your Apple ID.

•           The refund amount.

•           Your current delivery address.

if you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service

I can't transfer to my sister for example so she can buy something over there for me and send it.

If your sister is in the US, send her the gift card. She can redeem the gift card to her US account and spend it as the two of you (hopefully) agree. Study this first: What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

To your other point, Apple gift cards are not "real money." The real money was spent by your Mom-in-law on a US-based gift card, which is only good at Apple and only in the US. Diplomatically tell her to find a more appropriate gift.