I am very sorry about that, min.
I know how I felt when my own mother told me that she wouldn't come to my wedding. She ended up coming, though. No other witnesses came, but I think she didn't look too bad for her return-visits who happened to be a lot of my guests. Or she just didn't want to miss her only daughter's wedding. I don't know for sure, but she was not nice after wedding until I had a miscarriage.
Are you sure that the elder couple will be there? For my wedding some witnesses said they would come, but they didn't. The only one, who came, was a parent of my student who didn't know I was DA'd. As soon as she found out that I DA'd, she had her daughter quit piano right away.
Min, invite a whole bunch of your mother's RVs. It is a good witnessing. After my wedding one of our guests, who knew the situation, told me that she felt so sorry for me, and shocked how cold and unChristian the witnesses could be.