What may come across as "lame" excuses to you are obviously valid enough for the OP and a personal thing. Who are we to judge?
No one knows the type of relationship she has with her brother/SIL. She stated they weren't bothered by her not attending. Weren't hurt by her reasons etc.
I think it's been blown way out of proportion.
Remind me never to decline an RSVP in the future, unless my excuses meet the stringent criteria of the host!
Me 32, He 35, DS 14, DD 2