AIBU to think this is an inappropriate relationship?

Exactly this. Many split families take turns and the arrangement is every other weekend - because that’s the best time!

You’ve never had a Friday night cuddled up watching a movie, or a cinema trip on a Saturday afternoon, or a farm shop breakfast on a Sunday, or ice skating or Saturday night watching Gladiators or The Masked Singer. That’s normal family life with a 9 year old and you have no experience of that. You just do the start of the school week, which is just school, home, repeat.

You’ve kind of allowed this tremendous bond that your son has built with his Dad and Step-Mum - they simply spend way more time with him and do more things.

I wouldn’t have it. Tell them the new arrangement is every other weekend. Otherwise I think you’re in real danger of your DS choosing to live with them in a few years.

Oh sweetheart I genuinely feel bad reading that. Not because this woman is doing anything wrong but that I can hear the pain (and if I’m honest jealousy) in your post. To your son this woman is his family. She’s helped raise him since he was 3, he’s the mother of his sister and his step MOTHER. She won’t ever replace you but in his eyes she’s very very important to him. You need to change your mindset and be happy that he loves this woman, there’s nothing inappropriate and there’s room for both of you. If you can’t get over it maybe therapy could help you. He will always love you regardeless, there’s room for both.

I don't understand why he can't go on holiday with you, as well as with his Dad - why does it have to be either/or?

And how does his Dad take him away for 4 weeks each September - doesn't he have to be in school?

I do get that it's hard and stirs up strong feelings (my ex has remarried so son has a step mum, and I really struggled with the 'grandparents' even though intellectually I can see they're good for him). But you've got to put your son first and this relationship sounds lovely, not inappropriate.

This is not inappropriate at all. It's lovely they get on so well together. Are you sure you're not jealous.

You need to spend time with your DS at weekends and build in quality time together doing what he enjoys. Less time going out with your 'mates'!

Your DS enjoys time with his dad and step mum as they sound attuned to him and make him feel part of the family. He knows you would rather be on holiday sitting my a pool with your 'mates' than be on holiday with him.

I wouldn't be surprised if in time he will vote with his feet and stay with his dad full time.

YABU it’s no wonder he feels close to step mum she sounds lovely and has made an effort. You sound jealous and a but of a dick tbh!

Quite honestly, it sounds like you have a mental health issue. It sounds like you have dropped the ball massively, comments like you have never taken your son on holiday, your child's friends know his dad and step mum but not you, your son doesn't now want to go on holiday with you... It all sounds like there's a lot more to this then you are saying. You are going to get nowhere by throwing a tantrum that your sons step mother has built a loving and stable relationship with your child. She isn't a temporary girlfriend you can claim shouldn't have contact with him, she's a permanent fixture in his life. Grow up and start acting like a sensible coparent and perhaps you might repair the relationship with your son. Trying to destroy his safety net is only going to backfire on you. As you can see from the other comments. It's not them. It's you.

She sounds lovely and dotes on your son. No-one can replace Mum, ever. I would be delighted his stepmother and her parents are so kind to your DS x

Oh, OP
I think you are reaping the consequences of your choices.
He is still young enough to rebuild your relationship with him. This is important. Don’t waste time blaming his step mum.

Rather than looking at it in a way that you want her to back off, you need to do what you are able to influence which is your own relationship with your son.
You never have any downtime with him, which suited you when he was little but this is the knock on affect of that. You don’t have the closeness that you hope for and she does have a strong connection with him.
Adjust the arrangement- maybe start with at least one weekend a month with you. And parents make decisions about holidays, not children. They think they want to but it is not best for them.
Lean in to your relationship with your son, strengthen that and you won’t feel as threatened/bothered by theirs.

I can’t believe you don’t get any weekends either your son! Start working on the relationship with your son , he’s slipping away. Also his step mother is his family don’t be stupid.

Your son is so lucky to have many people who love him in his life.

Should anything happen to you, you should feel reassured that he would be well taken care of my his step mum and also her family as well as his dad.

I don’t let his dad have him for dad to not even be around.

No. You don't let his Dad have him. You share custody. And your son and his father have a right to be in his other's lives. And it shouldn't be at your whim.

I am a step mum, although he’s nearly 21 now, 3 when we met.

I was his one person allowed to go to hospital with him when he had an operation during Covid, his choice. I completely understand that it hurts, but as step mums we can care about the child and want as well for them as you do, no agenda.

I can see why you are jealous, but I wouldnt call the relationship inappropriate.

She is a lazy narcissist of a parent by her own description and thinks a biological link means more than day to day love attention and dedication . That boy knows what he has in both houses and he knows his home is with dad and stepmom . Who in their right mind goes away on holiday and doesn’t take her child until they are 9 .

Step mum sounds amazing, and Dad sounds very reasonable.

No it reeks of jealousy.

I have always felt it takes a community to raise a child. Your son is blessed with birth parents that love him, but also an extended family that treat him as one of their own.

Whilst it is a bit natural to feel a tad jealous of the bond that your child has with his step Mum, just be happy for him that he's so loved and was made to feel part of an extended family rather than being second best, like many other children are.

There is absolutely nothing wrong with anything you've stated that she does. She clearly sees your son as a child of her own, which is preferable to a them/him attitude, which would have hurt your son.

I feel very sad that you can't see past your own feelings and see how good the situation is for your son.

This is everything you would want in a step-mother.

This doesn’t sound very nice to be honest.
You have to meet your child where they are. I’m not saying everything should be geared towards kids, but you really should be thinking of holidays suited to him, not the other way around.
I’m not surprised that DS prefers to go on holiday with his dad and stepmum. He feels safe with them and they do things he wants to do.

You usually holiday with mates? have you ever been on holidays with your son?
stepmum sounds lovely, I understand why you are bitter, her parenting sounds more involved than yours.

I’m sure this is a painful thing for you to witness but it sounds like she’s an involved, caring step parent. Nothing sinister in what you’ve described.

You should think about changing the arrangements. If your son only sees you on schooldays but sees his Dad and stepmom at the weekends, it follows that he's doing fun things with them and never with you, so it's bound to affect his view of all of you. Likewise you should try to arrange a holiday that he would like so that you can build up your relationship.