Keeping a happy mood around an ill person-Not letting him know of the passing of a relative:[1]

If one is sick [or old], and there is worry that letting him know of the passing of a relative [or other individual], and breaking his heart, can lead to worsening of his condition, he is not to be told of the passing. People who are around him are not to show any signs of mourning; they are not to tear their shirts, cry, or eulogize in front of him.[2] They are not to comfort him, and those who do so are to be quieted. [This applies even if a parent of the sick person passed away, and even if he needs, and is able, to say Kaddish on their behalf.[3] Furthermore, one is not even to eulogize or cry regarding another person who passed away, even if it is not a relative of the sick person.[4] If however the person is not sick to the point that letting him know of the passing can lead him to danger, then he is to be told, and he is to follow the laws of Shiva. The Rebbe did not let his mother know of the passing of his brother during the Shiva, and kept it hidden from her for the rest of her life, for over 12 years.]

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[1] Michaber 337:1; Miseches Semachos

[2] Wearing Shabbos clothing: A mourner may even wear Shabbos clothing in front of the ill relative in order so the patient does not suspect that he is in mourning. [Chamudei Daniel, brought in Pischeiy Teshuvah 402:1]

[3] Beis Hilel 337

[4] Shach 337:2; Bach