I believe Betta macrostoma is a protected species in Brunei itself. Not sure what other species exist in Brunei though. In any case, wild caught fish are a big no-no in most cases.
Bringing Bettas back to Singapore will require the usual plastic fish bag with some atmospheric air or oxygen pumped in. The bag ought be a little limp, not super firm like when you buy fishes locally. Place it in a styrofoam box or a cooler box and check it in as luggage and not carry-on luggage. You will probably need to declare the fish at the Bruneian customs and later on in Singapore, just in case.
If in doubt, contact the Brunei embassy here in Singapore, before your sister goes on the trip. Ask for their customs requirement regarding live fishes and whether you can take them out of Brunei legally.
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