Miller, 32, has become inseparable with the 21-year-old model for some time now and has been pushing her friends away, including model Suki Waterhouse, to spend time with her, reported Femalefirst.
"Sienna is relentless about wanting to spend time with Cara. She seems a bit obsessed," a source said.
Delevingne's intimate friendship with Miller, whom she was caught kissing last year, may have caused her split with actress Michelle Rodriguez.
"When Cara was dating Michelle Rodriguez, Sienna was always getting involved and it caused an issue. She was always kissing Cara, which Michelle wasn't keen on," the source said.