Parents and Grandparents attending the wedding of their children and grandchildren during year of Aveilus:

Shiva:[1]

Parents: It is permitted for the father or mother of the Chasan or Kallah to participate in the wedding even during Shiva.[2] This includes attending the Chuppah, and being present by the meal and dancing, although they are not to eat during the meal, and are not to partake in the dancing. They are not required to do any Shimush by the meal. They may wear Shabbos clothing.[3]

Grandparents: Some[4] write that grandparents may participate in the Chuppah if it is taking place outside the wedding area. They may not participate in the meal.

Shloshim & Year of Aveilus:[5]

It is permitted for the parents and grandparents[6] of the Chasan or Kallah, to fully participate in the wedding even within Shloshim.[7] This includes attending the Chuppah, eating by the meal[8], remaining during the music and participate in the dancing.[9] They are not required to do any Shimush by the meal.[10] They may wear Shabbos clothing.[11]

[1] Gilyon Maharsha 391 and 392 in name of Kneses Hagedola in name of Mabit 2:99; Igros Moshe 2:169; See Kinyan Torah 4:121; Nitei Gavriel 16:16-18 and Shut 1 p. 743-764; Pnei Baruch 20:10

[2] The reason: As it is their day of Simcha and the Chasan and Kallah will be saddened if they are not present, similar to the allowance of a Yesoma. Now, although this allowance only applies after Shloshim, nevertheless this is certainly considered like a Davar Haaveid and even worse, and hence we permit it even within Shiva. [Igros Moshe ibid; Nitei Gavriel ibid]

[3] Nitei Gavriel Teshuvah 1 based on Rama 391:3 regarding Shushvinin

[4] Nitei Gavriel 16:17

[5] See Aruch Hashulchan 391:5; 391:10; Zera Emes 3:169; Maharashdam 202; Gilyon Maharsha 391 in name of Kneses Hagedola; Gesher Hachaim p. 186; Igros Moshe Y.D. 2:171; Pnei Baruch 20:17 and Poskim in Pnei Baruch 20:20 footnote 49; Nitei Gavriel 16:5

[6] Aruch Hashulchan ibid

[7] The reason: As it is their day of Simcha and the Chasan and Kallah will be saddened if they are not present. [Gesher Hachaim ibid]

[8] Other opinions: See Gilyon Maharsha 391 in name of Kneses Hagedola that he is not to eat by the meal

[9] Nitei Gavriel 16:5 footnote 7 in name of Poskim; 20:10 footnote 29

[10] The reason: As they fall under the allowance of marrying off a Yasom, as brought in Rama 391:2. Now, although this allowance within Shloshim does not apply to an Avel of a parent even by a Yesoma, and even by other relatives it is a dispute, nevertheless this is certainly considered like a Davar Haaveid and even worse, and hence we permit it even within Shiva. [Igros Moshe ibid; Nitei Gavriel ibid]

[11] Nitei Gavriel Teshuvah 1 based on Rama 391:3 regarding Shushvinin