Regular poster, NC due to very outing details, some minor details also changed.
Bit of background here, my MIL and my Mum decided to co-host a baby shower for me a few months back at my MIL's house. With my MIL's permission, gifts from family members who couldn't attend in person were mailed to her house.
My husband and I unfortunately came down with the flu and the shower had to be cancelled. Once we were well again we went to visit my in-laws and they gave us all the shower presents for our son, to take back with us.
Three months later we found out one of my Aunts had sent a gift card for the baby shower for a bit under £50 to my MIL's but we had never received. My MIL didn't recall receiving one so we assumed it had been delivered to the wrong address or lost in the mail so my aunt contacted the company to enquire about a replacement. They refused to replace it on the grounds that someone at the address it had been delivered to had used it.
My husband asked his Mum if she recalled any gift cards being delivered around that time from that company. It turned out that they were expecting a gift card from the same company for an item that my FIL had returned. They had mistakenly thought the gift card from my Aunt was for his return so had spent it. Then they cheekily thanked me for the meat grinder they bought with it. 
I explained it to my Aunt and she said something to the effect that it was on my in-laws then to replace it, which my husband and I agreed with.
The gift card would have been addressed to my name, so it was a pretty bad mistake, but mistakes happen. So we figured they would offer to replace it in cash or gift card form. We are not well off and actually need to buy something for our son that we'd hoped to use that money for and they have the money to easily replace it.
4 days passed and they not only didn't offer to replace it but they didn't even apologise for the mistake. On the 5th day my husband asked his Mum directly by FB messenger when they would be able to replace the gift from my Aunt. His Mum read it and never responded.
My husband's theory on why she's behaving this way is because her birthday was shortly after the birth of our son, we were so busy and dealing with being new parents and my PND that we forgot to send her a card or get her a gift. We did tell her happy birthday though.
Before all this mix up happened we were planning to get her a belated gift to give her when we see her this weekend. My husband thinks she's butthurt that we didn't send her anything for her birthday and bitterly feels like we owe her so she doesn't need to replace the gift card. She has form for almost never ever admitting she's wrong or has made a mistake.
The gift card was from my Aunt though for our son! Not for us even and certainly not for my in-laws to spend on themselves from my Aunt they've never met. 
We don't feel like we should have even been forced to ask for the money let alone be ignored when we did. I guess to some it's a small amount and maybe they'd let it go but it's not a small amount to us nor to my Aunt who sent it and I feel my in-laws have been incredibly rude.
So my AWBU is would it be unreasonable if when we show up at the in-laws this weekend if they don't offer to make things right that we just pack up and leave again? That's how my husband has proposed we deal with them.