Your experiences of family therapy?

Hi all, firstly sorry if this isn't the right forum for this discussion, if you want to point me to another forum please feel free.

I'm worried about my SIL and my husband's side of the family. Sil has a 18 mo daughter and is pregnant with her second. Even before kids, Sil had a difficult sort of personality, she's always been quick to anger, holds grudges and in my opinion can be quite selfish. That said, we used to mostly get along great.
Since having her daughter, sil has become more and more hostile to family members to the point where we're all walking on egg shells around her in case she has a major meltdown. She has a very strict sleep/feed schedule for DD to which she is a slave. Certainly schedules work for many families, but in this case when we plan a dinner or more often decide to do something on the spur of the moment, sil accuses everyone of disrespecting her schedule and purposefully trying to upset her.

She is rude to everyone and I find myself avoiding her, which is really sad because our kids are the same age and we live close by. I want to have a good relationship with her but I'm not sure what to do about it.

I thought of seeing a family therapist and perhaps convincing my parents in law to come along. Obviously it's ideal if we all go but I can't see Sil entertaining the idea. She's very much of the opinion that its everyone else's fault.

Her second baby is due in a few months and is just going to get worse, isn't it?

Can anyone tell me a bit about family therapy? I've never seen a therapist before.

Thanks everyone xx