Accessory to an Alleged Crime

First, there are no grounds for an oath because Reuven’s claim is not based on knowledge but on suspicion. There is a short list of cases in which oaths are made based on a claim of doubt, and this is not one of them.

P'ninat Mishpat (575)

Various Rabbis

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266 - Authority to Rule Based

267 - Accessory to an Alleged Crime

268 - Replacing a Problematic

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Second, even if Shimon did what he was suspected of, he would not be obligated to pay. Reuven’s wife is described as stealing the valuables. After she made an acquisition in regards to the laws of theft, the object was stolen, and Shimon’s role in helping her would not make him a thief who would be obligated to pay. While there are grounds to claim that Shimon acted as a mazik (damager) by enabling his sister to hide the items, it has not been substantiated that she would not have been able to do so on her own. Furthermore, once the objects were stolen, the main damage had already occurred, and making it less likely that Reuven would recover them is not direct enough to be considered damage. (E.g., if one obstructs the path of someone chasing after a thief, this is not direct damage which obligates one to pay.)
Furthermore, Shimon says that his sister had a right to take the objects [Ed. Note – it is not clear from the question based on what grounds]. Since there are no witnesses that she took the objects, if she admitted in court that she took them but that it was justified, she would be initially believed. Although Shimon’s witnessing of the event is like witnesses of the seizure in regards to himself, in which case, she should not be believed it was justified, Shimon also says that he knows that she is justified. Even if it is wrong in the first place to seize property due to a debt, that still does not turn the person who seized into a thief.
For all the reasons mentioned, beit din should not make Shimon swear that he was not involved in the hiding of valuables from Reuven’s house.