Need advice on preventing my 2 cats from fighting.

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Will go straight to the point. A few weeks ago, one of my kitten ran away from home and was missing for 3 weeks. I managed to find it somewhere behind an alley, and brought her home and got her cleaned up. When I put her next to her brother, they didn't seem to get along and wanted to fight. I thought maybe they couldn't recognize each other or something. But when I put her next to her mom, they start fighting. It's been over 2 months already and they're still fighting whenever they see each other. I've separated them, and find it more tiring to do more work in terms of cleaning up. So if any of you out there have a perfect solution for me I would be very much highly appreciated. Thank you.