loved her / didn't love her.. *spoilers*
my impression was that Tamsin definitely cared and had feelings for Mona.. though the word "love" is probably too strong to use.. it was pretty clear that Tamsin had some emotional problems, probably because of her parents who didn't care for her, and also because she had a bad relationship with her sister, thus feeling lonely.. this may also explain why she considered her to be "dead"... and because of these problems, she was probably afraid of getting emotional or intimate with people.. and that's why she ultimately got scared and ended her relationship with Mona... that's my take on the film