Is it okay if I shoot my SIL (Vent)

Dh had an fight with his sister over some issue of her constantly bugging him for game help and  the fact that she arranged a photo shoot for the family shot - considering in her 2 sisters schedules (she asked them when was easiest) and then phoned him and told him that at 6 at night we had to be at the photographer as this was the only time we could have the shoot - his sisters dont work and have kids older than 5 - so getting them ready is easy, us on the other hand work until 5 - my kids are 5 and 1 - who gets difficult around 6, and live 50 minutes away from the studio - so in short a major issue for us as the only way we can actually get there in time is if Hubby and I leave early from work - which is impossible. He decided he had enough of her and removed her from his bbm's as he was frustrated with her constant bothering him and fighting with him because he had a problem with her lack of consideration and expectation for him to jump and roll over at short notice.

So she starts a tirade to me via bbm - which  in each message I told her to leave me out of this argument as it has nothing to do with me. So she goes and post this on face book wall - bearing in mind that she is also friends with some of my family members - including my mom.

"Don't f*** your mother through the a**just because you don't like your siblings!! Is it you siblings fault If photographers has their own rules?? Be pissed off at the people who deserve it! If its in-laws, your problem, not mine!!!! So much for family saticking together! I wish my dad was still alive!!!! He would have sorted the in-laws out....without taking it out on his siblings!!!!"

My in-laws is my mother - who happens to be on here on her  holiday - that she flies down to visit us once a year for a six weeks. That is the time my mom gets to see her grandkids - so naturally I make sure that the kids spend as much time as they can with her during the time she is here.  MIL gets to see the kids every other weekend the rest of the year.

So naturally I am stumped at why she is trying to start a fight with ME and my mom - who has done nothing wrong and DH is fuming as she had no right to post that on facebook - of all places.

I sent her a message to remove that post from facebook, and appologise for her disgusting outburst.  But honestly besides for that I actually dont know what to do. Well what I want to do is never go near her or her kids again - but as she lives with my MIL, I still want my kids to see their grandmother. But right now I am still fuming and its taking all of my control not to step into Mamma Bear mode.

Thank you , I have reported it and blocked her and so has DH.

I will be letting DH deal with the rest.