Most siblings go through a period where they just aggravate each other. I have 4 boys & they are best friends & worst enemies! I think it just comes with the territory. Whenever mine start being really mean to each other, I make them go do something where they have to work together. Take the trash out, clean up the yard, clean up all their toys, etc. I make them hug it out. Generally, it makes them mad at me & then they secretly plot my demise. But they are getting along & working together towards a common goal so I'm ok with it lol.

Yes I have a 12 year old and 4 year old !! They can be best friends one moment and then the next the 4 year old is crying !! They are my boys , lol ! When their sister was here she was in the middle so it was 11, 7, 3 she acted like a buffer between the two. She could do the older things with the 11 year old but still liked doing the younger things with the 3 year old. Now the age difference is causing problems.
Me and my brother fought a lot but we are friends now.

My girls are 4 years apart, 10 and 6. My 10 yo is, (sorry darling...) sneaky. My 6 yo (again, sorry hon...) a drama queen. My 10 yo will push her little sister's buttons, I catch it time and time again. She'll wait til her sister is wound up about something, and then, just, push her a little further. I don't agree with 'fighting it out'. I really don't. I prefer to catch it motion, and then call them each out on their behavior. You, you *know* your sister is in a bad mood. You're taking advantage. Yes, I know you 'just' did that. And any other time, it'd just spark a fun time of teasing. Right now, all you're doing is pissing her off, and you want that, cause then she'll do something serious in retaliation that will get her in trouble. I'm fairly sure she doesn't think of it exactly that way, but she thinks its funny to push her sister over the edge into hysteria. So, don't do it, because I'll catch you, and believe me, I don't think it's funny. 

I really can't add anything new. I especially liked the comment that mentioned explaining to your son what it means to be a big brother. That's worked well here. conversely, I'd try to start explaining to your daughter how lucky she is to have him, and how she should treat him. @chaosmom had some good ideas.

Also, try your best to ensure they both get alone time with you and your husband. I'm in education,so I know how hard that is this time of year. I also know how hard it into not yell when you've been keeping your cool all day with other peoples' kids(I used to teach middle school and high school with kids who had emotional disabilities, so believe me, I get it). It does make a difference though.

I agree with everyone; it is part of being a sibling and the transition from summer to a more scheduled routine might be shaking things up right now.

Omg mine bicker non-stop. Ds11 and DD 8. I keep telling myself that it will stop eventually, I'm a firm believer that you gotta let them work shit out on their own mostly. I only step in if it gets physical or if I overhear something wildly inappropriate. For the most part this works for mine. I sometimes have to remind DS that he's the older brother, no matter what he nerds to loom out for her. Etc...so one day he looms at me and says "mom, she's my sister and she gets on my nerves. But if any other boy said to her what I just did, there'd be a problem"

I remember my sis and I growing up, we were 22 months apart, had to share a room, clothes, toys. We fought like crazy at times. But we always had each others backs and still do. We got into it so bad one time, she grabbed a high heeled shoe and came at me with the heel end, I grabbed the phone (the old bell rotary kind...HEAVY) well we both had some bruises, lumps and scratches...lol. but that was the last time we even argued.

I guess my point is all siblings do this type of thing, and most of them outgrow it and end up with a lifelong friend out of it, people they can count on no matter what

Watches the magic 123 video years before being a parent as used it for work. One thing that struck me is when something happens between both kids they both get a time out. His point being that many times the "victim" has done something to provoke the other kid that you did not see, and when you only punish the "offender" the "victim" gets away with it and it reinforces that It really made me think of all the times I provoked my brother and only he got into trouble. Yup I was that kid lol. So with our boys when they are fighting they both go to time out.

ugh.  2 years apart--they will take turns picking on each other. in the next year or so, your dd will be the one terrorizing him. give him a chore he hates when he is being mean. and if the little one is egging him on, give her a chore she hates, too. my son hated to vaccum. my daughter hated to sort socks. guess what i did very little of for a few years? better yet, give them a chore that absolutely has to be done with cooperation. and praise them on what a good job they did for their chore. my aunt did this with my sister and i. when my uncle got home, he always knew if we had been little shits because the house was sparkling clean. "look what a great job the kids did cleaning the house" (wink, wink) "wow. you guys really worked hard today."  we got sick of cleaning everything, and would just glare at each other, instead.