I have been hearing chatter at auctions and other points of sale that if you buy a sibling of a higher value coat/fur type you are more likely to have it in a brother or sister as well.

Is this true?

It is not true. What you see on the breeding screen (parents, grandparents, great grandparents) + planet is all that matters. Any additional relatives, including ancestors, do not influence the outcome.

Thank you Asymmetrical,
Melangel brought up a huge peeve of mine and it is cool that you (Mossms staff collectively) clarify that stuff early on so there are less shady selling practices out there.