If your sister's daughter(s) have a history of stealing from you, common sense should tell you not to entrust your valuables to your sister or at least in a position where your sister(s) daughters have access to said property. As per the "devaluation" of the antique due to it's restoration, that could be construed as actual damages, but, of course, that's up tp the judge to decide. i would still include that monetary damage in my lawsuit backed up by a professional's expert opinion on how "devalued" the item would have become.
Remember, "stealing from you" is a criminal offense. Civil courts deal with monetary damages. You're unlikely to get your "property" returned in civil court. of course, if you have a police report of the theft and a subsequent conviction, it would make your case much stronger in civil court as the burden of proof shifts from "beyond a reasonable doubt" to "more likely than not" (or something like that but less restrictive and easier to prove).
If you already have a strained relationship with your in-laws, and they know what your options are (criminal and civil liability on their part), dropping this matter and chalking it up to experience, MAY be a BIG STEP in showing a good-faith effort on your part to better the relationship. Just don't store things in their garage anymore. Get a storage unit, if you need to store things.