To expand on what Christacular was saying, the main ethical/philosophical argument against eating the eggs is really one of the strongest and reasonably flawless arguments that exists (in my humble opinion) for veganism and animal rights--and that is in the inherent right one being to not be consciously exploited by another, no matter how "nice" the exploitation is. While this may come across as the most "hardline" argument for veganism, it is incidentally also the most solid. I have found through my years of research on the topic, that the Utilitarian approach advocated by Singer in the Bentham tradition, and followed and advocated by what seems to be a large segment of vegans, is a slippery slope and extremely difficult to establish.
Personally, contrary to my tongue-in-cheek signature, I am not nearly "hardcore" enough to argue at length to try to give you any particularly good reason to stop eating eggs from your sister's farm. But, I thought I would give you a bit more of a primer on what argument could
The only other thing it sounds like you are looking for is someone to tell you about how unhealthy
eggs are. Quite frankly, I find extensive debate about the healthfulness of a vegan vs. nonvegan diet extremely agonizing. For every article, doctor, website, study, etc. that advocates a vegan diet, there are at least ten more that don't. Obviously, one reason for that is there is a lot more research on the non-vegan diet; but either way no matter what you say, someone will always have some "evidence" to "prove" you wrong. I.E. look how many times the dreaded soy debate
comes up on these boards, and everyone has a different opinion, and many different sources for their beliefs. If optimal health alone was the only reason I was vegan, I would be extremely wary of admitting it because it seems like it would be almost impossible to prove that it is the healthiest or even one of the healthiest diets. It is much easier to me to say, "It's healthy, good for the environment, and anyways, I am opposed to the unnecessary exploitation of other beings for my personal palate."
OK, now this is getting too long.