Q. My mother was Niftar on January 11, 2011. When is the year of avaylos over for me. Second, am I allowed to go to a family Purim seuda with about 15 people (all family including cousins and their children)?
A. January 11 corresponded to the 6 of Shevat (until sunset). Avelus is only 12 months, since this is a leap year, it will end the 5 of Kislev at nightfall.
It is certainly better to invite the family to celebrate the Purim Seudah at the ovel's home as some Poiskim prohibit his attendance outside the home. (Nitey Gavriel, Avelus 2,31,4 and Purim 78,14 - Mishne Halochos 13,229).
Some Poiskim permit attendance outside the ovel's home if no music is played there. (Yalkut Yosef 695,12 - Nishmas Ysroel, Purim 371). Other Poiskim permit attendance even if music is played. (Divrei Soifrim 40, fn. 12 and 17 in the name of Chaim Shoal).
Horav Shlomo Miller Shlit"a permits the attendance of the ovel, if the music was not requested by him.
Rabbi A, Bartfeld as revised by Horav Shlomo Miller Shlit"a