It's interesting how one's perspective changes as one gets older and encounters situations in life that they never thought they'd encounter.
I had the opinion (and to some degree still do) that having non-Jews by the Seder is wrong. Not wrong as is "it's forbidden, you're violating a commandment, " but wrong as in "IMHO, it's not appropriate." Now, I've been by s'darim where non-Jews were present, and I must say that they have always been curious, respectful, and have always followed the requests of the hosts.
But the story of Pesach is the story of the redemption of the Jewish people from a slavery in a situation and manner that was unique to them at that time. Certainly other cultures can appreciate and identify with a set of rituals surrounding a story of redemption from slavery or freedom from oppression; but the method used on Pesach, with it's unique symbolism and history, is as unique to Judaism as a Juneteenth celebration is to the descendants of African slavery in America. Unlike most holidays, the celebration of Pesach is more personal, since it is we, the Jews, who were rescued from Egypt. Having other people there is, well... it just seems out of place.
Such was my thinking for years. I've never had a non-Jew by my seder. They've been by my Shabbos table, in my Sukkah, dipped apple in honey with us on Rosh HaShannah and even by my Pesach table (during the non-seder meals), but not at the seder itself.
Of course, it's easy to hold an opinion in the abstract. It's when the situation hits home that you really begin to consider how important your preconceived notions are.
Case in point: my brother. My brother recently married a girl who is not Jewish. She's a very nice person (I couldn't see him marrying someone who wasn't a nice person) who is kind and caring. She's pleasant to be around and is respectful to everyone, even in the knowledge that the family wasn't thrilled with the wedding plans.
Well, this will be the first year since the wedding that we will be having a seder that she will (probably) be attending. We could always have not invited them (and their son), but we chose to do so anyway. Eeees and I figured that if we don't invite them, there is the definite possibility that my brother will not go to a seder at all. Despite his marriage, he is still obligated in the mitzvos of matzah, marror, reciting the story of the Exodus, etc. as anyone else is... by not inviting him simply because I may not want his wife present, I may well cause him to not fulfill these mitzvos at all.
In addition, we want to actively hold out the possibility that he will be chozer b'tshuva. I firmly believe in the rule that you attract more flies with honey than with vinegar. Eeees and I believe in maintaining strong contact with my brother and his family in the hopes of showing them that the door is always open to his return to a observant lifestyle. We respect and interact with his wife and son in the hopes that maybe they will see the beauty of such a lifestyle and want to lead one too (granted, the chances of this happening are not very likely, but you never know). Shunning them, however, was never considered -- by shunning them you are, in effect, closing the door to the possibility of his every observing the mitzvos again.
So, Eeees and I made the choice to invite them to the seder that we are hosting this year (we're only hosting one... we're going elsewhere for the first night). I have no doubt that my sister-in-law will be the perfect guest. I have no doubt that she'll look forward to the opportunity to have her son participate in the rituals and celebrations that are a part of his heritage (even if he isn't Jewish by our definition - and even if he's too young to understand any of it). For all this, I can put aside my discomfort.
You never know which action will be the one that will influence a person to make a decision in their life. You never know what little action may influence my brother to start keeping a mitzvah -- any one. If having my sister-in-law and nephew over for the seder tips him even slightly in the direction of deciding to stay home from work one Shabbos, or think twice about eating something non-kosher, or putting up a mezuzzah in his home, then it's worth it.
(Besides, my nephew is sooooooo cute. :) )
ADDENDUM: I just want to make it clear (since, as Baal Devarim pointed out, I didn't in the post above) that my relationship with my brother is not based on our hopes to have him become observant. I have other family members with whom I have absolutely no chance of ever influencing them to become observant -- and nonetheless we maintain contact with them, have them over at our house, etc. 'The relationship is not about trying to convert people to observance of the mitzvos -- it's about family and friendship.
That being said, it is no secret that we would love for our brother to become more observant - he knows it. But it's also not the basis of our relationship. He knows that we accept him -- and his wife and son (and any future children) -- even if he never becomes observant at all.