Are your nephew and niece adults? Most people would be hesitant about moving into their fiancee's home when having minors also living there, a ready-made family, so to speak, plus huge responsibilities and liabilities. 👦👧
I don't blame your fiancee either for not wanting to share a home with 2 other people also living there. What if he wants to relax just wearing boxers/underwear, etc. 🛀 Or like most people, he wants some privacy, or alone time with you?
Other than that huge issue, if it were just the 2 of you, I see no reason for him not to want to move in a home already owned, saving tons of money, plus the hassle and wait of shopping and buying a home.
Once you're free of having your niece and nephew living there, and you meet another prospective fiancee, I'm sure he'll jump at the chance to move into a home free and clear. Good luck 😊