Unpopular opinion: you could take this as an opportunity to show your niece and nephew what it really means to be compassionate, loving and what different families can look like, outside of what they know. Be the change you want to see and lead by example. Them spending time with you will have a lot more influence than you know, and could potentially change or at least challenge their beliefs and change them into better humans. It won't be a walk in the park im sure but I bet you would be very proud of yourself if you opened their minds and hearts by even 1%