Hi! Bonding is something that I’m not the best at either since my rabbit is very aggressive. It seems to me that Arnie is the leader and Siggy isn’t okay with it. Now this might be surprising but females tend to be more aggressive and males more calm although it always depends on your rabbits, so Siggy is mad and wants all your attention. I have a few questions that are going to help me out to know, so I can help you out:
Do they get their own food bowl?
How often to you guys play together?
Do they have their own litter box?

One thing that might help is for you to set up a time just for Siggy to be pet and then a time for just Arnie to be pet. I also used to have a rabbit that would hump y female a lot and she would do the same things as Siggy does to Arnie. I learned that if she gets to be alone for a little each day it helped her out a lot and after a little alone time she would come back a snuggle with August (the male who would hump her). Maybe try a little area with a hidey house where Siggy can go and be alone for a little while? Let me know if this helps

Hi! So I would say it’s best to have their own litter box, food bowl etc but you can still put them next to each other. As long as theirs two. Siggy might feel better to have some of her own things It’s good there is no fights over food, it’s a good sign! It’s good that Arnie is bonded to you! Awww so cute, that’s for sure a good thing=) It’s fine that you don’t handle them, most buns don’t like it anyway.
Yeah I could never get my girl out either lol. One thing that might help is to get your boyfriend to take out Siggy, put her next to you (you don’t need her on your lap or anything) and you start giving her treats (doesn’t have to be store brought you could try fruit or oats, which is my bunnies fav once she’s at ease with you start slowly petting her. Make sure to come from the side so she can see you and doesn’t get scared. Do this every so often and see if it works.
It’s also best that once she calms down, to set aside a time for you to go and play with the buns for while.
It’s good that she gets to be alone! Sometimes they just need to get away from the boys for a while!
Let me know if this helps and how everything goes!

The humping isn’t a problem… but only if the one being humped is OK with it. It sounds like they aren’t, so it’s worth paying attention to.

Have there been any changes in your household? New pets? New people? New cleaning products?

Any new scents can trigger dominance issues.

Also, how much space and exercise do they get? Ideally you should not have to pick them up to let them out to exercise, as that can damage trust. Siggy might be protective over the cage because it is “their” space, and she is right! Anytime you reach in there you are invading their safe zone, so if you have to do it anytime they come out to play it can trigger aggression as she is protecting their zone.

Most store bought cages are too small, even for one bunny, so I’m wondering if they are having issues because they are too crowded.

My bonded pair has two litter boxes (side by side), and two food bowls, even though they often each out of the same one.

Also, if Siggy is suddenly aggressive, it could be due to a medical issue. Anytime there is a sudden behavior change it could be due to a rabbit in pain. This can also trigger issues in bonded pairs, so it might be worth a vet visit to get them both a check up.

I also think Bunnybuzz gave some good tips on earning trust from both bunnies.

I think you can also try a quick “couples therapy” bonding session to see if that helps. Try either a car ride together or popping them in a basket or box and vacuuming around them. You can follow up with banana on both heads to end with good feelings all around.

DanaNM: Yes I think that’s a really good idea for the litter boxes and food bowls- that way they have a choice