Agreeing with jaxsue.
OP, you mention that you would see what you could do to be at your sister's wedding if it were on another date than around your milestone birthday, *and* excluding a major holiday. Is the reason for excluding major holidays due to the increased cost of travel in those windows, or because you prefer to celebrate holidays yourself, without traveling to family? I have no issue with either reason, and am asking only to better understand your perspective on the present matter.
It sounds as though you may "feel" you "should" make efforts to be at your sister's wedding, and are not entirely comfortable not going without a "valid" reason. If that is so, flip the thought process around so instead of looking for a justification (to yourself or others) of why you won't be there, you ask yourself "why would I be there?"
That isn't meant as a snarky question, but given the lack of any significant visits from or to family over more than a decade, and what sounds like a challenging relationship, or at least difficult communication patterns with your sister, if the sole reason you have for why you would attend her wedding (or any family event) is "because we are related, though not emotionally close, and I would *like* a better relationship" the follow on question would reasonably be "if I do attend this wedding/event, is there a reasonable hope that the relationship would be aided, or would I be meeting family members desires/requirements, but the relationship to be no better than now - and now is not very good."
I agree with many points made by PPs, just think fully understanding your own "feelings" and expectations will help limit the potential to second guess your decision, whatever it may be, and avoid self doubt, if you are having any.
Also, I definitely agree that it would be better to not mention your milestone birthday as a factor in any decision not to go to the wedding (you may know that's the reason (or not), but stating it is no more necessary that saying "I don't feel like it" rather than "that isn't possible").
And, whatever significance you attach to any birthday, milestone or not, is entirely up to you, no justification needed so long as you are not requiring anyone else to share that sense of significance. I hope you find a meaningful way to celebrate this, or any event which is of importance to you.