Any bond is compatibility - character and mutual liking - these two just didn’t have it.
You can certainly try another of your sister in law’s pigs and see what happens.
It would be best if you try and find a way to have her and a new pig live side by side for a few days before trying to bond them.
This allows the new pig time to calm and settle and for them both to get to know each other between the bars before going through the stresses of bonding.
If you can’t do side by side for a few days beforehand, that is fine though. I know it’s not easy with them being outside in a hutch