HELP!!!!II want whats best for my little girl but i dont know what to do?

my daughts father went to prison when she was 18 mo.his family had nothing to do with her during his 2yr abseancesafter he was released he asked if he could take her to see his mom he had always been a good dad i agreed it was 2 yrs later that i found her he took her to his sister in a nother state she and her husband still have her shes now 6 her father hasn't been there at all it doesn't bother him to hear her call his brother in law daddy but it kills me to here her call his sister mommy she knows im her mom but my ex sister in law intentionally doesnt answer my phone calls loses my mail ect they take very good care of her shes very much loved but shes mine and they had no right taken her but when i get the nerve to insist on coming to get her she melts my heart when she has something to say and says mommy to her she then looks at me like shes introuble or confused i dont want her to hurt and i think taken her would hurt her more than leaving i haven't seen her in a year what doido