Revealing the lower legs in front of her mahram
*Please appropriately reference this fatwa to: www.fatwa-online.com, thankyou!*
Question: Is it permissible for a sister to appear in front of her brother - revealing her lower legs [from below the knee] and arms?

Response: It is not permissible for a woman to display [anything] in front of her mahrams and [other] muslim women other than the [well known] areas of [her] beauty; Amongst them [that which] Allaah (Subhaanahu wa Ta'aala) said:

{...and not to show off their adornment except that which is apparent, and to draw their veils all over juyoobihinna (i.e. their bodies, faces, necks and bosoms) and not to reveal their adornment except to their husbands, or their fathers...} - Soorah an-Noor, Aayah 31

And the [well known] areas of beauty are:

The hand and [the neck area] where the necklace is worn, and [the wrist area] where the bracelet is worn, and the upper arm where the bangle is worn, and likewise the feet, and [the ankle area] where the anklet is worn.

So these are the only areas which are permissible for a woman to display in front of her mahrams and [other Muslim] women.

Shaykh al-Albaanee
Fataawa ash-Shaykh al-'Allaamah Muhammad Naasiruddeen al-Albaanee fil-Madeenah wal-Imaaraat - Page 245
Fataawa al-Madeenah - Page 96