To understand why dd1 is often invited for weekends away and dd2 is not?

[D0oinMeCleanin]

DH thinks it is favouritism.

Dd1 is almost 9. Dd2 has only just turned 5. Dd1 is fairly well behaved and mature for her age. She sleeps, she listens, she understands no, she might sulk for a while but she is past throwing full on strops.

Dd2 does not sleep. Ever. Midnight is her typical bedtime. She'll still still be up and raring to go at 7am. On bad nights you can expect to be awake until 3am or beyond with her. She has no fear of anything and thinks nothing of running off somewhere and not telling anyone. She doesn't listen to anyone. If she is made to listen (one example would my mum telling her to move away from the edge of a cliff only to be told NO by dd2 and then physically attacked by her when she had to drag her away) Any attempt of discipline is met with violence or a tantrum lasting hours.

So AIBU to think that until she is older and calmer and has some regard for her own safety, then expecting relatives to care for her for a weekend is unreasonable?

If you only had 1 child then your husband would let DD1 go off with relatives. I don't see why DD1 should miss out because she has a younger sister. There will be a time when DD1 has left home and DD2 gets to go to relatives without DD1.
If DD1 goes away you can then spend more time on DD2 plus reinforce with her the need to behave and do as she's told when she is invited out.

I have similar problem. I have dd aged 8 and ds aged 6. Ds is a handful and I would not expect other people to look after him overnight . He gets up at 6am no matter what time he goes to bed, still occasionally wets the bed and is very particular where breakfast is concerned. Having said that he is much better behaved with other people and there have been no problems at school. DD has a friend from school here today and I have had to police pretty much all activities as ds trys to spoil things if he doesnt get his own way. DD gets invites to her friends houses but ds very rarely gets invited to play dates.On the very rare occasions he has been to someones house the reports are always favourable so in a way it makes me a bit sad that his behaviour when children come to our house is putting off kids from wanting him to go to their houses to play.

Put it this way dooin, what harm can pushing for assessment do? It will either be a yes and a dx or a no and life goes on. I buried my head in the sand a bit with my DD and a such the first two years of her school life have been rough leaving her with self esteem issues. Not every difficult child has SN but it's worth eliminating and the earlier the better IMO.

i agree with fate.
they won't diagnose her with something she doesn't have, but if there is some kind of SN then it will allow you to access support for her.

there really is no reason to wait and see, in my opinion