Is she being a bitch?

Hannah • Mummy to a beautiful baby girl 💕
Two of my friends have very kindly offered to organise my baby shower - which is this sat! They have invited everyone who I asked them to including my older sister. She messaged one of the organisers during the week to say she couldn't come and according to my friend the message was just a bit weird and came across as if she didn't believe in baby showers...which didn't really surprise me as she said to our younger sister 'baby showers are so not my thing'...I was with my sister today and she hasn't mentioned anything - didn't say she couldn't come etc...the only reason I know is because my husband is away this week so one of the organisers is staying with me and mentioned it...she thought I knew... Am I right in thinking my sister is being a bit of a bitch with not coming and not telling me she's not coming??