If the mother-in-law holds a wedding for her youngest son and the eldest sister-in-law doesn't want to go, she can first understand her reasons and communicate effectively, and respect her thoughts and feelings. If the eldest sister-in-law cannot attend due to personal reasons or other special circumstances, she can communicate with her mother-in-law in advance and explain the reasons. If the eldest sister-in-law is unwilling to attend because of her bad relationship with her brother-in-law or other personal reasons, she can consider discussing this issue diplomatically and seek a solution. Finally, if the eldest sister-in-law insists on not attending the wedding, the mother-in-law should also respect her decision.