If my co-sister-in-law (husband’s brother’s wife / Orppadi) passes away and within 4 days my grandson’s Ayushya Homam falls, can it be performed? Or should it be performed on another day?

By grandson, are you asking about your daughter’s son (Dauhitra)? It is not clear in the question. If it is your Orppadi, you will have ten days of ritual impurity (Theettu). It cannot be performed during the period of ritual impurity.