Is it alright to have an OC that's the elder sister of Atem? (However, she's completed repulsed by the idea of pharoahs marrying siblings and relatives) Or would this be too Mary Sue-ish?
Honestly speaking, being blood-related to a canon character immediately holds up the yellow card for Mary Sue.
However, if your OC, even if she's Atem's elder sister, is well-written and has enough depth to support her I think you can make it work anyway. It's a tricky thing to write, actually, but I know you can do it!
Thanks for you question and I hope it helped!
Well, like Admin Magica said, having an OC related to a Canon character - at least a main one - is pretty risky. For the most part their backstories are already defined and so is their family, not leaving much room to "fit" someone else, without warping considerably what it's already established. Most people do consider sudden "new" blood relatives of Canon characters as Mary Sues, but it also depends on which character.
AND just like Admin Magica said, I personally believe that if an OC is extremely well written and developed, there's a lot of otherwise known "dangerous Sue territory" stuff you can end up getting away with. There are exceptions this too, of course, like new Millenium Items and other types of absurd things, but really, it's actually possible to make nice balanced characters even with aspects like those, if YOU know how to work with it. At the very least, people may ignore the slightly Sue-ish part of your OC if everything else about them is believable and likeable. Like I mentioned before, I've seen OCs who were related to Pegasus, for example, and they were still pretty good =)
Going more specifically to your question, I think it may be a little hard to make a sister for Atem, specially an older one, but with a lot of care and attention you probably could make it work. It could be possible for him to have a sibling, considering that not much is known of his family even after we find out about his past (we only know about his father and all), and from what I know, many Pharaohs usually had more than one child. You don't have to get into that whole deal about several wives they could have and consequently children, so I guess that by what we know, you could "squeeze" her in the Royal family xD
Just don't forget to do a lot of research of how the Pharaoh's children were raised and treated, specially considering that your OC is a girl. My knowledge is a little rusty right now, but I think that even if she was the first born/elder child, she'd most likely not have the direct right to the throne being a female and all. I may be a little wrong though, they obviously had a lot of respect for the Queens, so either way, she'd be well treated and pampered; However, don't let her "outshine" Atem in some ways. She might be the oldest but HE is the prince and will become Pharaoh in the future, so most likely the bigger part of the family and servant's attention and priority in almost everything would go to him. I don't know how deep you plan on going about his particular aspect (their family life and all), but it's a important thing to note, if you wanna keep it realistic with the Ancient Egyptian royalty patterns and not Mary Sue-ish. Then again, of course, you don't have to make everything COMPLETELY historically accurate, as the Anime/Manga aren't 100% correct either, so it's okay with her not liking the relatives marriage stuff ^^ If anything, you don't even have to mention it, as it doesn't hold a very important part in the Canon story for Ancient Egypt. But you can keep that dislike of hers if you want to like, make it a little bit more realistic with real history but still fitting inside the Anime/Manga's boundaries.
All in all, it's difficult and risky, but if you make your OC the right way for them to be well-rounded with everything else and research a lot to make it work, you can do it! If you have any more doubts, feel free to send other questions and also lastly submit your OC for critique so we can give our full input in everything you worked on :3
Good luck and thank you for asking!