relative-in-law

NOUN /rɛlətɪv_'ɪn,lɔ/
15 LETTERS · STARTS WITH R

relative-in-law is a valid 15-letter word worth 19 points in Scrabble and 23 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
R1E1L1A1T1I1V4E1-0I1N1-0L1A1W4
1 + 1 + 1 + 1 + 1 + 1 + 4 + 1 + 0 + 1 + 1 + 0 + 1 + 1 + 4 =19 PTS
DOUBLE WORD → 38TRIPLE WORD → 57
MEANING

What does relative-in-law mean?