The newest version of FlightBriefing (1.4) includes SIGMETs on the map for easy identification. They may be tapped to display the SIGMET text.
Furthermore, fresh weather reports (METAR, TAF and SIGMET) can be downloaded from within the app without the need to re download the whole flight briefing package. On the weather page, pull down and release the list, or just tap the refresh symbol in the upper right corner.