Igor,
There are three main types of of the Order of St Sava.
First. Obrenovich type 1883-1903 with red robed saint and Milan I monogram on reverse
Second 1903-14 red robes and and 1883 on reverse
Third post 1914 green robas and 1883 on reverse.
The Third type is by far the most common and in lower grades cased good condition examples are not that hard to obtain and are not hugely expensive, as far as I am aware this type seems to relatively free from being copied
They are always silver gilt and enamels, there may be privately made gold examples, sometimes on heavily toned examples the silver-gilt can look like silver.
A general point for collectors always best to buy from from a reputable dealer/auctioneer, while they may not be quite as cheap as some on some sites at least you do have some kind of guarantee if the piece is not as described.
As your family is of Croat origin probably the type 3 is most likely to have been awarde to Croatians during the inter-war period, the majority of type 3 pieces a manufactured by Huegenin.
Best of luck in your collecting venture and please dont hesitate to ask for help on this forum, there are many of us who can give good impartial advice.
All the best,
Paul