Catelyn arrives at the first stronghold to the Vale, where she finds her favorite (and only) uncle, Brynden "The Blackfish" Tully.
As they continue to the main castle (the Aerie), Brynden tells Catelyn that Lysa may be less helpful (and sane) than Catelyn remembers. Lysa has had a lot of trouble bearing a child and her son Robert is sickly.
When Catelyn arrives at the Aerie (led past three more strongholds by Mya Stone, another illegitimate child), Lysa yells at her for bringing Tyrion here. Lysa just wants to be left out of this fight. (Tyrion gets placed in a holding cell.)
And little six year-old Robert starts to get nervous, so Lysa breastfeeds him.
Robert also wants to have Tyrion thrown out of the castle – and we mean literally thrown out, which is how they execute people since the castle is on a mountain. So, great: Robert's weird and cruel.