]While in their cradles, we planned the union."

I think it's very reasonable to think that two sisters, sitting and gazing at their infants, might say "Wouldn't it be wonderful if they were to marry?" I have no problem believing this part of Lady Catherine's statement. However, we have no idea of whether this thought or idle wish carried on as the children grew up. It could have been just idle talk, which grew into a certainty for Lady Catherine after Lady Anne became ill and/or died.