SIL had an affair and run off with the kids

[BackoffSusan]

Earlier this year SIL started behaving rather recklessly - lots of drinking at home, disappearing for over night benders with my brothers best mate, saying she wanted to stay in a hotel on her own and she hated being "just a mum". She'd only been married 6 months but I thought she was having a breakdown or depressive episode. She then instigated a separation from my brother in April. They both went their separate ways, and then in June it transpired she was in a relationship with my brothers best mate and been having an affair. The best mate then moved away in September - about an hour and half away from the town where she and my brother lived. Then last month she disappeared with the kids. Up to this point my brother had been seeing the kids twice a week with an overnight stay on the weekend.
My brother has no idea where she is. We assume she has moved in with the new bloke but she and new bloke have blocked everyone so we can't contact any of them. Brother has submitted all the paperwork for a child arrangement order but has been told it can take a really long time. I tried messaging her and ringing but she didn't reply, and then blocked me. I only asked if I could send the kids their Christmas presents or see them. I don't really know what else I can do. Social services and police have been very unhelpful. It feels like they just signpost to each other and now she has left the local area it's hard to know what to do.

And just to add, it may well be that there is much more to this than meets the eye and that OP isn’t fully aware of the surrounding circumstances, which may explain what looks like nothing having been done so far, after all, we’re not privy to all if the info on here.
However, if we take the OP at face value from the details that are included and what we DO know, then mum and kids certainly would be classified as missing.

For reference, I’ve just spoken to my dad on the phone a matter of hours ago, he could ring police now and say he has concerns about my welfare based on nothing, and they’d create me as a missing person until I was located.
The bar for a missing person creation is surprisingly low, because ultimately, police forces don’t want to answer awkward questions and be held accountable for fobbing people off in the unlikely event that whoever the concern is about turns up dead.

Difficult without a address bet is police who can trace them eventually