I am an ex wife, and a girlfriend.
so I shall hopefully help with both sides.
from the EW POV - my XH & I split over 3 years ago - he was abusive & had an affair yada yada yada. the woman he was with in that instance caused an awful awful lot of trouble with us, awful lot - I hated her with a vengance & demanded that she was never to see my child, or indeed if she did I had to meet her first - XH then said well I want to meet your partners (fair point I might add in retrospect so be prepared for that one) my beef was she was a young girl who tbh was out for splitting up a marriage (rough time etc we were having it's all by the by now, and events that have unfolded after have proved she got exactly what she wants & has now moved onto one of XH's other friends. He on the other hand over the 18 months, and subsequent 2-3 months post her realised exactly what he had done, by at every opportunity putting her above our DS (who was 2 at the time). I had hatred towards her yes, but also towards him - he is a good dad when it suits him - it annoyed me so much that he prefered to be with the woman rather than his child - I would cry on mums shoulder asking her to tell me what was sooo bad with my child that he didn't want to know him - why he would rather be with her than his son, the list goes on. but I realised - and you prob will do and believe me it's a bloody hard pill to swallow but he is never going to be the father my son deserves all the time he puts someone else above his child. for me my son is first above work, bills DP etc etc everything my child is no 1. end of discussion.
that realisation was v v harsh but needed. I feel sad for DS that I choose a shite dad. Moving on thou, XH now has a new partner - she I suppose is pleasant enough - not met the girl, and tbh I have no desire to now - I have to trust that XH will keep DS safe when he sees her for the W.E. DS is of an age now (5 soon) that he can tell me if he likes someone or indeed if something is wrong. from this POV I monitor his behaviour v v v closely and tbh anything slightly suspect I am straight asking XH (in the way I would when we were married - ie to be abrupt with him gets him defensive, to enquire and suggest that it's not a problem etc gets me more answers). So far DS seems to like the girl - but he's more keen on talking to my mum about her not me - which is fine, he's talking to someone. from this POV i'd suggest if/when your kids do meet the lady then they have someone to talk to about her as they might not want to talk to you as might not want to upset you. My issue is with the GF's parents trying to take over and dictate to me how I should raise my son & take over with XH & the GF - XH has been told, that it's me & him who are parents the GF's parents are not to be involved until DS has developed a good relationship with the GF first. I hope they will do - i'd sooner DS be happy with his dad and GF than not tbh. this is something you will reconcile in time - but it will take a lot longer than 3 weeks - and tbh as I say 3 years and 2 girls later i'm reconciled with it.
from the GF's POV however (DP has a DSS) I get on well with DSS, and enjoy our time, I encourage XH to talk to DSS, & his ex), I don't like the way that sometimes from an outside POV, that DP's EW uses DP as a baby sitter when it suits her - something which considering how much effort he puts in I find heart breaking on his behalf - if he was a shite dad/not bothered etc then i'd have no prob in saying to him step up but he's not. so from that POV, it might not be the GF - I've only said once about this to DP, as tbh it's nothing to do with me, the only thing I ask is that everytime we have DSS and I'm around (as i'm not always - it's good they have 1 2 time) that the EW agrees for me to be there - so far she's been fine - and again if DSS wasn't happy then i'd not be about.
If your XH's GF has kids she should understand that it's about the kids - it's a fine balance yes but ultimately about the kids, you all need to work together, cos at the end of the day to fall out only stops the children. Grit your teeth - it is hard, very hard, it's not nice, and lord knows XH & I have out times still now, but mostly if the new GF can get XH to do more with DS then i'm all for it.
Trust me if you were to step back & allow the XH to have the children on his own fully without you about - would he be the same or would he not?
sorry it's so long, hopefully it helps? possibly not. who knows just rambling.