Getting married in portugal

No, not me unfortunately

My sister is planning on getting married in portugal, and there are a few issues that she needs to investigate. Most websites mention that one of the parties must have spent 30 days in portugal before applying for a marriage license.

Now, both parties live permanently here in the UK, however about 6 months ago they bought a flat in portugal which they plan to use as a holiday home.

Anyway, how do they stand legally for getting married in portugal? Any such marriage would be a civil ceramony if that makes a difference...