The sign for "in-law" is generally done by just adding "LAW" after signing the person.
For example:
"BROTHER-in-LAW" is a combination of BROTHER and LAW. That means it is a "compound" sign. You generally get rid of the first part of the sign LAW. Some people even get rid of the last half of the sign brother.
BROTHER: (This is a popular version of the sign "brother.")
0:00 / 0:00
LAW:
0:00 / 0:00
This is how I recommend you sign "brother-in-law"
Sign brother, then place an L on the palm:
0:00 / 0:00
Now, check this out.
BROTHER-IN-LAW:
When you see "in-law" as part of a phrase such as "brother in-law" you might see it drastically shortened to include only the first part of the sign for brother and the last part of the sign for law.
SISTER-IN-LAW:
When you see "in-law" as part of a phrase such as "sister in-law" you might see it drastically shortened to include only the first part of the sign for sister (girl) and the last part of the sign for law.
IN-LAW: Here is an "English" way to show "in-law." You just sign "in" and then sign "law."
But please note that most of the time the sign "in" is dropped when you are signing compound concepts like "brother in-law." (see above). I don't know of anyone who would actually sign "IN" plus "LAW"
Animation: IN-LAW
Animation: Brother in-law (version 1)Animation: Brother in-law (version 2)
Notes: