Comparative form of sincere: more sincere(adjective)
Wiktionary.org : Text is available under the Creative Commons Attribution-ShareAlike License
Use "sincerer" in a sentence
"I think that they're honest and I think they're sincerer and I think they're kind."
"They were bolder, sincerer, more true-hearted, more upright, and more united than those whom they overthrew."
"In this episode, Fanny describes Louisa: never was a girl constitutionally truer to the taste of joy, or sincerer in the expressions of its sensations, than she was: we could observe the pleasure lighten in her eyes as he introduced his plenipotentiary instrument into her ..."