mrsgravy - are you conflating the OP and kathy? The latter hasn't linked to her outfit.
kathy - length sounds fine but might I suggest a jacket or shrug?
To be fair it really depends how laid-back everyone is about you, my brother's MIL was absolutely vile to my bro's poor ex at his wedding, but she's quite, erm, opinionated anyway. I'd go for a messy bun, demure courts, natural makeup and (most importantly) if you have a +1 your best accessory will be a man.