It's normal for cats to be hissy when they 1st meet, they are very territorial I would be more worried about what the white cat has if it's semi-wild. I take it the it has'nt had any needles ect, if you can't get near it ? It could also have FIV ect, which if it is aggressive and bites one of yours would'nt be good at all. I can understand that you feel responsible, so could you not have some kind of shelter outside for white cat ? then have food, water whatever it needs out there. Just some ideas for you.