Your husband holding the older three children to different (higher, stricter) standards than the youngest is bound to cause issues between the siblings. Does your husband want all of his children to be close and get along? If he does, then he needs to hold ALL of them to the SAME standards, and punish them in the same way for the same offenses, etc.
I can tell you from personal experience that favoritism helped ruin my relationship with my sister. My sister is most definitely the favorite. Always has been and always will be. She was allowed to do A LOT more than I ever was, and it was clear that my mother favored (and still does) favor my sister. It has put a huge wedge between us. My parents are still playing favorites to this day and they have done way more harm than good, in terms of my relationship with my sister. I could go on and on about the things that they have done to show favoritism towards my sister, and vocally supported my sister not having to communicate with me if she didn’t want to. My sister and I have not spoken in years and I don’t see it changing.
I did want to mention something in connection with the above.
My next-youngest sister and I came from a very dysfunctional household. One of the more egregious dysfunctions was that I was “dad’s kid,” while my sister was “mom’s kid.” Mind you, we’re full siblings. It’s that personality-wise, I got along best with dad, and she with mom. That’s not too uncommon, or even a bad thing. What was a problem was that both parents recognized and encouraged this mindset–our mother by physically abusing me and actively demonstrating how much she loathed me, and our father by belittling my sister and being nasty to her. At the same time, our mother made a big deal as often as possible about how beautiful, intelligent, talented, etc my sister was (and how I wasn’t), while Dad spent a lot of one-on-one time with me but never with my sister. Dad wasn’t around as much, and Mom was careful not to be really egregious in her treatment of me while he was, so got the brunt of a lot of this stuff: if she was frustrated, she’d hit me; if she was having a bad day, it was my fault; if I didn’t understand directions or move fast enough, it was because I was deliberately defying her, not because I didn’t understand, was a bit paralyzed by perfectionism, or simply didn’t pick up on social norms as quickly or intuitively as my sister.
As you might imagine, my sister and I had a very rocky relationship when we were kids and teens: we hated each other. My sister, though younger, tended to taunt me into a foaming rage, and then, if I struck out, would threaten to tell our mother until I groveled enough to persuade her not to, as telling our mother would mean I’d get a beating. Once we were old enough that physical abuse wasn’t an option, she could just tell our mother that I’d said something mean, and that would get me punished severely, even if she had said the same thing to me. On the other hand, I could generally fight back by getting Dad to “cancel” my grounding/extra chores/whatever and by getting him to get really angry at my sister for doing this stuff and scream at her about how if she were just a nice, easygoing person like me, he’d like her as much, too.
It was bad, and it set us up for a very nasty, resentful dynamic.
After we were separated for years, though–didn’t see each other for about six years and spoke rarely–we were able to meet on more equal footing as adults, especially once we had done a lot of work on ourselves via counseling (her) and a support group and program (me). As it happens, thanks to us both recognizing the serious flaws in both our parents (even the favorite ones) and having that cooling-off period, we’re now as close as sisters could be. Neither of us have contact with our parents anymore, which is for the best for both of us now. When some sort of crisis arises with them (our mother drunk driving, or dad in the hospital with something serious), we handle it together as a team. I can only attribute all that to grace and to our both finding some very good people to help us through, but it did happen, and for that I’m more grateful than I can possibly express.
So, long story short: it is possible, long-term, for siblings set up in this sort of relationship to end up having a good, close relationship later on. It will take a lot of hard work on both parts, though, and nothing is guaranteed. Parents should never try to create that adversarial dynamic, of course. It usually does a great deal of harm, and no good at all. I think I can say with certainty that if it weren’t for grace and some good, wise people, my sister and I probably wouldn’t be speaking at all, and each would still see the other as a monster rather than as an inevitably flawed kid placed in some very nasty circumstances.