I think the consensus is, OP," Yes, YABU". Why didn't you ring when you got the letter, feign surprise, say of course you're looking forward to seeing her, but DH is at work all day, how about another day/time, or is she happy just to see you etc?
You haven't said how far she is travelling or how, or indeed if this is a one day visit or a "visitation" lasting a week or two, so without that information, it is hard to see why this is such a problem.
It would be interesting to hear how you feel about it now.