There's nothing you can do, she'll learn or she won't.
If she wouldn't normally go see her family just for the hell of it then she's probably going there to see him. If she does mess around with him then she will probably feel guilty,if she feels very guilty then you will probably be able to detect it in the way she avoids certain things or by acting differently than normal. From there you could allude to matters of fidelity or scare the living shit out of her or whatever, if you want to have fun with it then you could suggest that she bring her cute friends in for three ways,then you could bone other chicks and let her off the hook at the same time ...Personally ,I think that people are generally too fucked of for monogamy....
no
Your the one who stated, "She's totally cool, so I trust her." Well it doesn't sound to me like you do and with that I have a problem with your relationship, as if there's no trust there can't be much of anything. As a small comfort perhaps; this was from a relationship book written by one with a Phd.... In most every failed relationship there typically comes a time when the couple will try it together again. What happens in most every case is they will break-up again and probably for the same reason they broke up the first time.
Just give out hints, for example how are you going with your old lover and so on and show that your feeling worried that way she will tell you that there is nothing going on and will think more when she is comronted with him. It just like when my mother tells me strongly or effectivly not to touch the damn ice- cream it works more on me and I think about it when she is gone, its not a faded thought of should, I or should I not, its a NO!
I tell you what...maybe her motives are straight up...but I don't think taking up with an ex is cool at all, especially if she is leaving you out of the equation. THe only way I would consider having a friend of the opposite sex is if they were also friends with my husband (and this would also apply even if we weren't married) and I would NEVER take up with an ex.
Let's just say...she might be cool and all...but don't get too serious with her until you get past this problem, because your trust in her has been shaken. It will take time to build it up again.