Let's start out by saying I have one slightly unusual aspect of my life, I am a known sperm donor. I've donated to more then one couple trying to have children (through AI, nothing sexual). Our agreement makes it clear I am not a parent for these children, I won't interfere with the actual parents choices in how to raise the child or try to demand custody; however, I still care about the children and try to stay involved in their lives as much as the parent's allow.
I visit and play with the kids as often as I can, I'd say I have a kid visit at least every other weekend on average. I skype with kids who are further away, and have babysat closer children. I've arranged for larger donor-sibling get-together's a few times a year so all the kids can play together and my parents have even met one mother and her daughter, at the mother's request. I still know my boundries as a donor, I would never claim to be a parent to these children or violate the wishes of their parents, but my point is that there is still a real connection with the kids I'm allowed to know and it is a non-trivial part of my life.
My sister is not supportive of my being a sperm donor, which I think has more to do with complicated sibling rivlary reasons then her disagreeing with sperm donation in general. In any case, while she hasn't explicitly forbiden me in words, she has made it quite clear she does not wish for me to share the fact that I'm a sperm donor, or that I have a relationship with the kids, with her own children. For the record her husband doesn't seem to have strong feelings either way on this subject.
This already goes against my preferences, as I believe very firmly in open and honest communication with children. I think sheltering kids from facts 'until their older' leads to many potential negatives, including kids feeling lied to later, kids viewing world in more black-and-white manner that leads to treating people who are different worse, and a lack of openness in communication which leads to older children making stupid mistakes because they were afraid to come to their parents for advice. In short, were it my decision, I would explain my being a donor, to the level each child is capable of understanding, immediately. However, I also am very aware of the importance of respecting the parent's decisions and don't intend to go against my sister's wishes even if I disagree with them in this situation.
However, I'm feeling worse about keeping this a secret from her eldest son, who is now a teenager in high school and quite capable of understanding the concept (though, admittedly, his keeping the secret from his siblings is not as gaurentted). Lately he has made jokes about my never dating (I'm aromantic) and specifically asking "when am I going to have cousins", which makes not sharing the fact that at least from a biological sense he already has some feel more like a direct lie. Furthermore he has already overheard about my visiting with or babysitting donor children and has asked about who the chlidren were before. I have had an extensive history of volunteering with children in general, so usually all that is needed is a slight diversion from the question and he will end up presuming that the cihld in question is just another child I'm volunteering with; but again it feels dishonest to evade a question and allow him to make a (partially) false conclusion.
I have no intent of violating his mother's wishes by actually telling him I am a sperm donor, at least not until he is old enough that he deserves the right to make decisions independent of his mothers, and can be trusted not to say anything to his siblings against his mother's wishes. However, are there any options for me to mitigate the risk of his feeling I was lying to him all this time when he does learn (without placing the blame entirely on his mother for the lie either). How can I be as honest as possible, in communicate to him, or his younger siblings, that there is more to the story that I can't share yet, but that I intend to, without violating his mother's wishes or making him curious enough that he keeps asking his mother uncomfortable questions?