Eldest's Mum Is Coming To Visit And He's Terrified

My girlfriend and I are raising our two-year-old and my sister's six-year-old, after my sister's husband (a SAHP) died unexpectedly. We knew the husband was dying and the young master would eventually end up with us, so he'd had lots of trips to his cool gay aunties' house, was going to a school in our zone, etc. My sister- his mother- is stationed in Afghanistan but managed to wrangle some leave for a little after the New Year....The problem is that he wants nothing to do with her. He refers to my partner and I as his 'real mummies' and his mother as 'my other mum.' He's getting better at talking to her on Skype, and has been practicing writing e-mails to her, but there was a period where he wouldn't talk to her or about her, and would leave the room if we were talking to her on the computer.I've been saying that his mother will be coming in 2018 and he bursts into tears, saying 'I don't want to go, she's going to take me away.' He hasn't seen her in person for over six months and I think in his head he's conflated 'moving to a new place' with 'seeing Mummy'- after all, the last time she saw her he ended up coming to live with us.How do I calm him down? What should I be saying and doing? via /r/Parenting http://ift.tt/2l3FRJO