During the year of aveilus is it permitted to go to the engagement of a great nephew (my brothers grandson)?
There are different opinions to this. However he opinion of R’ S. Z. Auerbach zt”l is, that it permitted for the avel to go to say mazal tov, to taste something and then to leave. However if there is music or dancing then he may not be there at all.
Mishmeres Shalom 60, Minchas Shlomo 2- 96-12, Pnei Baruch teshuvos From R’ S. Z. 3, also brought in ohel Yackov Aveilus pg. 94.