I think it is lovely that an Adult son actually has aweekly viit and stay over with his elderly Mum. Sounds a lovely man.
you have him all the other six days and nights, why begrudge his Mum having his company the other day and night. After all, she has known him a lot longer than you have.
How dreadful to load on him that this is some sort of competion as to his loyalties. Surely, he has the absolute right to wish to keep happy relationships with his Mum and sister as well as at his home.
We hear far too many people on these threads complaining that their Adult children do not keep in touch. Yet, when one does (this man), we find his wife moaning.
So, he has not 'confronted' his Mum and Sister about non-attendance at a wedding. Such 'confrontation' would do no good, but could lead to arguements, enstrangement and lots of sadness - what on earth would be the point of that?
Perhaps the OP could spend sometime, trying to approach her Mother-in-Law to mend fences and re-build a good relationship.
I can only have praise for her lovely hubbie.