Hello Iceboy, welcome to the BB forums.
People should not be judged on their sexuality, everybody has a personality because appearance does not determine this, and to lose 63 kg is a massive effort and whether you have been questioned why this was your target, needs to be answered in full, especially as the relationship has been tenuous.
She may already believe you're gay and when you tell her isn't going to make any difference in how she looks after the children and remember kids are astute, they know what's going on and as parents, we often try and pretend, hide our hidden secrets but all in vain because they start to talk.
It's not going to make any difference whether you tell her now or after Xmas, except to prolong your anguish, and unfortunately other people also have their own family struggles during this time of the year.
As they say 'there's no better time than the present', the longer you wait the more difficult it will become.