I love my dead sister's husband

Pretty much the title.

Two years ago my sister passed from thymoma, she has twins who were 3 at the time. Her husband (Dan) handled it really well. He's since then been a model dad in every way. We became close, as I'd help look after the twins, help him out and generally be there. About 6 months into this an old friend of his visited and seeing us asked if I'm his wife, they weren't aware my sister had passed. Ever since then I've been seeing more and more of what I want from a partner, in him, and have even rejected others because of it.

I feel like telling him would end in rejection and ruining our relation. My mother (she's known him since he was 15, treats him like her own son) told him she wants him to start dating, this makes it even harder as he was opposed to it and still kind of is.

I'm not asking for advice realistically, it's appreciated but not needed I just wanted to let it out