It is difficult but sometimes there is little that you can do other than keep letting him know that the behaviour is unacceptable.
My DSC behave badly that their moms - and I mean badly! But at our house they are like completely different kids. We are strict though and have boundaries in place (such as no jumping on sofas, no hitting and swearing, bedtimes etc). Mom does not - she will laugh if DSS swears as it was funny when he was little - now he is nearly 9, not so.
The one thing I will say - and I am sure this is just our example and not ALL moms would do this but.....
When the kids were smaller mom used to phone us and ask us to pop over if the kids were not listening to her or doing as she wanted, she often gave them black bags and told them to pack their stuff and go and live with dad if they didn't behave and we were often portrayed as the strong disciplinarians. Then she would swear blind the kids only behaved at ours because they were scared of us??? They have never been smacked at ours (but mom does smack) the most is sent to room or time out elsewhere when little.
When ex told DH that the kids were scared of him he stopped involving himself in behaviour at her house and now he just explains to mom that she is not consistent and laughs so therefore they do not take her seriously and that is up to her - this has led to well behaved at ours and if we take them out and not so much at moms.
Thank you for your reply. They have been separated for a couple of years now and we see the boys every weekend. They are perfectly behaved for us and not for their mum. I think it's because we don't put up with bad behaviour, we nip it in the bud. I think as a single parent, she finds it easier just to put the tv on or give them something to eat. I understand that it must be difficult for her and I do want to help. Thanks for the book tip, I think an amazon trip is in order.
How long have the children been living apart from their father?
Chances are the eldest one is obviously affected more by the split and seems to be acting out. Surely your fiancé needs to sit him down and tell him this behaviour is unacceptable in school and with his mum, and support his ex with action to curb it.
I don't think it's a case of having more one to one attention in your house than his mum can give, it's more likely to do with taking advantage of his mums nurturing side. I will always take a softer stance with my DS whilst my husband can be very direct and strict. Boys need good male role models in their lives to set the boundaries and it's important that he knows there are boundaries expected of him even when his father isn't there.
A suggested read might be "Raising Boys" by Steve Biddulph. Quite an eye opener into the needs of boys if this is a new experience for you.
My husband-to-be (three weeks away) has two gorgoues boys aged 3 and 5. The 3 year old is an absolute angel and when the 5 year old is with us he is too... hower... when they go home to their mum and in school the eldest is very badly behaved. He tells people that his dad, my fiance, hurts him by pulling his hair and hitting him. Then when asked if its true he laughs and says "no, im just joking!" its not funny. I dont understand why when he is with us he is good but then not at school. is it because we give him so much one-on-one attention that his mum couldnt give him because she is by her self? why can we handle him but his school cant?