I'd rather have two competent relatives working with me than me having to deal with an incompetent CEO. I am saying that because I have worked with incompetent CEOs, who drove me close to the edge with their antics.
Two competent relatives leaving the firm is probably no more nor less disruptive than two friends/colleagues leaving the firm. The entire IT Security Practice of one of my former employers including myself as Acting Director of the Practice synchronized our notice of departure to take place on the same day. How disruptive is that? Needless to say, the CEO's idea of forcing us to participate in his lead generation contest on pain of getting fired - we were willing to generate the leads but we didn't want to participate in the leads generation contest because we thought it demeaning to us as professionals - didn't seem to him afterwards like the hottest idea since sliced bread.
You shouldn't hire anyone that you are not capable of firing. While the words "relatives" and "family" come to mind, so do the words "significant others" and "friends". And no one should be referring family, significant others and friends to the firm if they aren't prepared to accept that their family, significant others and friends can be fired for poor performance. I have a CEO acquaintance who fired his best friend - Their kids are playing together, even as we speak. I have no doubt that my little brother would have no compunction about firing me if I were on his team and I turned out to be incompetent - and that's just the way I want it and like it. And I expect my best friends to act in the same way. As the Russian proverb says "Friendship is friendship but business is business" And right now, we are doing business. As the Godfather may have said "Nothing personal" :)
You might object that if you run into one of the relatives, you could get into trouble with the other. You could get into the same trouble if these two people are good friends. And you don't seem worried about friends working in the same firm. If you are worried about friends working in the same firm, you might as well be your own employer and if one of your employees becomes your friend, you might consider firing either that employee or fire yourself from your own firm.
Large firms may have rules restricting family members and spouses from working say in the same department but it depends. I think we all agree that it may create an appearance of conflict of interest to have friends and family to do each other's performance reviews but there are ways to manage this issue including getting multiple reviewers to sign off on the performance review, or have the friends and family recuse themselves from doing the performance reviews.
What you want to do in the future is strictly up to you. You can live with it and stay. Or you can review your other options including transfer out of the team, group, department and departure from the firm. If you own your own firm, you can set whatever rules you want as long as these rules pass legal muster.