I’ve never personally watched Steven Universe, but my understanding (supported by my boyfriend who did watch it) is that the queer rep comes in three categories:

• Members of an all-female alien species have romantic relationships with each other. Aspects of these relationships are allegorical for challenges faced by queer people (for example, the “purist” aliens persecute the romantic aliens because of their relationships), but the characters in-universe don’t explicitly acknowledge the relationships as queer.
• Word-of-god from the creator that is not explicitly acknowledged in the text - for example, that a character is nonbinary.
• Under-the-radar references that would be easily missed by a conservative who didn’t know what to look for.

For that reason, Steven Universe might be an easier sell for conservative parents, especially for your sister. (I could see some parents getting upset at their son watching a “girly” show - presumably that’s not an issue for you here.)

I’ve watched all of Owl House and loved it. Spoilers below about the nature of the queer representation, for those who want to watch it (which I highly recommend!).

spoiler

The earliest representation I remember is a few episodes in where a mainish character mentions her two dads, which everyone treats as very normal. They don’t come up often, but you can’t forget about them either. Later on in the second season (I believe), the main character starts a same-gender relationship that continues to the end of the series and is much more visible on screen (hugging, kissing). I don’t remember the characters specifically talking about the queerness of the relationship very much, but they do come out as bi in a montage in season 3. Mentioning it because if your parents heavily monitor her media consumption, that might be a riskier bet.