No, there is no “gentle” way to point out that the one very striking example of nature’s beauty that always serves as a fond reminder of their deceased loved one has no connection whatsoever to that person. Why? Well, while you are trying to be factually correct in some fashion, you’ll also come off as a total Grinch. Besides, I’m not so sure you be certain you are 100% correct. There may be a bit of truth in what they see in those cardinals.
I’d encourage you to see their belief in terms of simple faith. Remember what one of the older sisters at Carmel told St. Thérèse of Lisieux: “I should think, child, you have not much to tell your superiors.” “Why do you think that, dear Mother?”… “Because your soul is very simple; but when you are perfect you will become more simple still. The nearer one approaches God, the simpler one becomes.” St. Thérèse–this “Little Flower” is a Doctor of the Church, mind you!–took the sight of snow on her clothing day as a particular favor sent to her from Our Lord Himself:
*Do you remember my telling you, dear Mother, how fond I am of snow? While I was still quite small, its whiteness entranced me. Why had I such a fancy for snow? Perhaps it was because, being a little winter flower, my eyes first saw the earth clad in its beautiful white mantle. So, on my clothing day, I wished to see it decked, like myself, in spotless white. The weather was so mild that it might have been spring, and I no longer dared hope for snow. The morning of the feast brought no change and I gave up my childish desire, as impossible to be realised…
After the ceremony in the Chapel I re-entered the Convent and the Bishop intoned the Te Deum. One of the Priests observed to him that this hymn of thanksgiving was only sung at professions, but, once begun, it was continued to the end. Was it not right that this feast should be complete, since in it all other joyful days were reunited?
The instant I set foot in the enclosure again my eyes fell on the statue of the Child Jesus smiling on me amid the flowers and lights; then, turning towards the quadrangle, I saw that, in spite of the mildness of the weather, it was covered with snow. What a delicate attention on the part of Jesus! Gratifying the least wish of His little Spouse, He even sent her this. Where is the creature so mighty that he can make one flake of it fall to please his beloved?
Everyone was amazed, and since then many people, hearing of my desire, have described this event as “the little miracle” of my clothing day, and thought it strange I should be so fond of snow. So much the better, it shows still more the wonderful condescension of the Spouse of Virgins—of Him Who loves lilies white as the snow. * from Story of a Soul, Chapter VII
Why would Our Lord not grant such a thing to your relatives to ease their grief? Who are you, to crush the comfort that comes from such a childlike faith? No, rather than to say it does not come from your relatives’ deceased loved ones, tell them it is evidence that those relatives are willing to intercede for those left behind so that God will send cardinals as evidence of their intercession, and encourage them to ask their relatives to include them in their prayers for all the needs they have from God. Encourage your relatives to also pray for those who have died, as we are all one Church, the living and those who died in Christ alike. Surely anyone who believes in a Heaven is willing to accept that all its good things come from our loving Father Himself. He did encourage us to ask for all things, and that we get into the habit of interceding for each other!
There might be an opening at some point to say that dying and rising in Christ is *better *than being an angel, since Christ didn’t die and rise again as an angel but as a human being. Sliding them over to the communion of the saints and reminding them to pray for their loved one and to ask their loved one to pray for them would be a good thing.
While we cannot ask for the intercession of just anyone within the liturgy, we can ask anyone, even someone who might be among the Holy Souls in Purgatory instead of already among the saints, to pray for us. If you use this belief in an afterlife already planted in your relatives to move them towards a more active connection with the entire Church, both those living and those who have died before us, that’s a step in the right direction along a path they seem inclined to travel.
In other words, be careful not to crush their faith but to elevate it to an understanding that recognizes even greater gifts from Heaven than they already imagine. I think that is both more charitable and more true, both at the same time.