First issue is you are not "confused about your emotions" you say you "feel angry and jealous". So that's the feelings clearly stated and it's OK to feel your feelings.
You can waste you time and energy telling yourself that "Your Feelings" make you a "horrible person" or you can own those feelings because you know that those feeling are directly related to what is happening in your DD's life.
I'm working on the assumption that your DD's treatment and situation is as you've stated.
If it helps I feel so angry when my DDs are treated badly that if I got the chance I would inflict physical damage on anyone who hurts them. I know lots of people will be unhappy that I wrote that but it's true, sadly these days I'm not as able to do it.
Several years ago I broke a guys nose in a market for slapping DD1 and I don't feel bad about it. Don't think he expected a woman with shopping bags to kick him in the face and brake his nose but my hands were full.
So I'm with you on the feeling angry but as for jealous I'm not sure about.
However, if you are of the opinion that her OH and his family have an inappropriate amount of influence and manipulation where your DD is concerned then I can understand you saying you feel jealous.
So are you jealous or are you "Scared" ?
Scared about the additional damage your DD may suffer from being in a relationship with this man and his family?
Are you also "Angry" with your DD? Angry with her for keeping herself in what seems like a TOXIC relationship ?
I understand that you are "Angry" about the treatment she has already 'Accepted" from this man and his family. Yes he and his family dished out the treatment ...
But by staying and continuing in the relationship your DD is "Accepting" the treatment they have already shown her AND she is also "Giving Permission" to him and his family to continue treating her badly as well as "Giving them Permission" to treat her even worse.
I'm guessing you sort of know these concepts even if you can't put them into the words I use.
If these are some of the things that are worrying you and upsetting your emotional balance that is most understandable.
Now what do you "DO"? AS most people have suggested there really isn't much you can "DO".
It is your DD life. These are her choices, and it's up to her what she chooses to so in the future.
Somehow we parents have to sit back on so many occasions and watch situations we know will leave our AC being hurt and hurting. How comfortable we are doing that depends on who we are, what's happening for us at the time and many, many other factors.
So if you are struggling to sit back and doing nothing (and not inflict harm on him and his family) I understand your frustrations.
Consider from now on talking to your DD in a different way. If it is possible say things like by staying you have "Given him and his family Permission" to treat you badly. Or put another way "Once people get away with bad behaviour they know they can treat you badly again".
One of the big things you can do is ASK your DD what outcome she wants from being with this man, his family once the baby is born. HOW does she think life will be for her and the baby and HOW likely is it that her life will be as she "thinks" will be.
IS there a POINT when his/their bad treatment will go beyond what she can cope with?
WHAT is her plan for that time?
Do you have a plan to help her if/when her world turns upside down?
Get a plan, where she'll live, how she'll pay for it, who'll look after the baby? Those sort of things.
And please do not even think the HE WILL be Reasonable, Pay, Be Kind, help with childcare or anything that is helpful. HE WON"T. KNOW this NOW, help your DD Know this NOW.
Get her to a Counsellor, into Women's Aid to talk to someone, or on there website.
If at all possible enable her to see that she is Very, Very unlikely to EVER have the life she thinks she will have with this man and his family.
Then IF she is happy with this Crappy life, really happy with her Crappy Treatment then you have to find a way to let her have her crappy life with this sh.. and get some counselling to help you cope with her choice.
Good Luck and I wish you, your DD and your soon to be DGC the very best of outcomes.