They do seem to come as a team. When Randall asked them to babysit, he asked both. They moved to California to help Kate with Jack.
Kate could at least consider asking. She doesn't know, but we and Beth know, that Rebecca is feeling not normal because neither Kate nor Kevin are asking her to help with the kids.
Given how she never went to a filming of Kevin's show and the way she behaved when he was a guest on the show, I'm thinking it is just typical behaviour for Rebecca. It's rare for Rebecca to give Kevin attention if she feels her other children need attention.