Everytime I go skiing I say those words.......everytime.

I submit that it is some sort of anagram. Your poor papa is trying to communicate something to you but he is afraid to come out and say it directly.

So, let's take the words, Remember, you are the youngest son and see what we have.

"Er, buy term heterogeneous yam"

or, "Merry Heterogeneous tube yam"

By combining the two options I have come to the answer. I think your papa is saying, albeit, hesitatingly, something about the purchase of dissimilar (tube) yams, perhaps, a lot of them, over a period of time. And to be merry about it.

I think that solves that.

Thank you. Thank you very much.

GRUESOME MENSTRUATE OBEY HER YON
Decoded: Watch out for women on the rag.

HEY GENEROUS MOMENTOUS BETRAYER!
Decoded: You've betrayed him, but you're still not a tightwad, so the news isn't all bad.

BUY THREESOME, GET ERRONEOUS MAN. Y?
Decoded: When you sign up for a threesome, you should find out the sex of the third partner.

MEET RESURGENT AMOROUS HONEY. BYE!
Decoded: Chicks on the rebound will really suck up your time.