Happy Birthday. There's something about the nature of birthday cards that if they have the name of a type of relative on the front (father, sister, mother-in-law, etc.), then they will have a gender-stereotyped illustration that is vaguely inappropriate for about 95% of the people who might possibly receive it. They're sent by people who fear the [left blank for your own message] cards, to relatives they don't know well enough to decide whether they would prefer something with an artsy photograph or a rude joke, on the logic that it demonstrates that you can at least remember how the two of you are related.