Hey! I never normally have much of a team (max about 8 horses) so I’ve never had this problem! But - I forgot to check the parents of the horses before I had exported them to the CK, and have no idea if some are full brothers and sisters, is there a way for me to check this is the CK before uploading them?

You need to check in your game, from the horse list in the ck sub menu. ‘L’ button next to their name. I tend to think the reason for this rule is somewhat redundant now anyway with league mode and no tff files.

I had to leave one out that did well because already hard a brother who wasn't even doing the same Distance or Hurdles and now the ttf files are gone might be time for Leon to look at.