First things first :
Your step-mother's behavior is suspicious as you describe it.
As you describe it you are being bullied into signing a legal document you are not at all confident it is good to do. This is very bad behavior on her part (at best) and very suspicious behavior at worst. You are quite right to have the document checked.
You would be entitled under European law to have any person you want examine the document and advise you on it. You do not need to ask permission or get your stepmother's approval.
Now your stepmother may simply be behaving in a way that you perceive as bullying, but you may be exaggerating it due to your own anxiety (or some other issue). We're just get your side of the story and sometimes that isn't a full picture. But that doesn't mean you should not be reasonably cautious, especially when signing legal documents.
How to do it.
As you clearly do not have much self confidence in dealing with what you see as a bullying personality you should bring a friend with you when collecting the documents to examine them.
This friend can act as a witness to how she behaves (and you behave) and can record the meeting on e.g. their phone if need be.
You clearly need emotional and physical support for the moment and that's the way to do it.
If your stepmother refuses to give you the documents to have examined then that in itself would be suspicious behavior and your friend can act as a witness to that.
Typically in European law you can nominate any lawyer to act as your representative in part or in whole. No one is entitled to refuse to deal with your lawyer under normal circumstances if you tell them that the lawyer is dealing with it. If you want to you can have the lawyer write to your stepmother and formally request the documents be submitted to them for examination prior to possibly signing them if you are happy with them.
No one, except a court or under certain circumstances the police, would be entitled to deprive you of the right to this representation if you choose it.
You are entitled to speak to a lawyer and discuss this matter without asking your stepmother's permission.
Anything you say to your lawyer would, and even the fact that you spoke to one, is something you would normally be entitled to keep confidential if you want. A lawyer is normally legally bound to respect your confidence and is legally protected under most circumstances (unless you both act in a criminal conspiracy usually) to prevent even a court from making them disclose details of what you say to them.
Note that you should normally get your own lawyer in this type of situation. Do not use the same lawyer as your stepmother for this issue.
Now, honestly, lawyers can be a nuisance sometimes as they can slow down things that need to be done. However your concerns are such that you should talk to one.
You sound like someone with possible health issues, either as a result of being bullied or dominated or (and please forgive me from saying so, but as devil's advocate) with extreme anxiety or difficulty dealing with any confrontation.
You may be suffering from depression (grieving after a death can cause quite extreme depression) which would be one good reason to suspect you have problems other than a domineering stepmother. I've had extreme depression myself after family deaths, so I'm saying this from personal experience. And it can take a long time to deal with this depression without help.
You probably need to consult a health professional about this.
In the long run you may be able to gain the skills and self-confidence or to reduce your anxiety (if that's an issue) and to function better in what may be an unhealthy relationship with your extended family.
You cannot deal with any issues like this in the short term, so you need to deal with the legal issue first - to protect yourself from possible exploitation by others.
But in the long term, I'd suggest the sooner you seek some medical advice on your psychological state (either as a result of being bullied or just a severe lack of confidence) the sooner you will start to improve. You might look upon these events as an opportunity to address the larger problem of how your living conditions and personality affect your quality of life and how to improve things.
After my fathers death, who payed for two life insurances (for my two step sisters), his heirs (in this case only my stepmother and me are over 18) have to sign this insurance in order for my sister to get their money.
There may be a translation issue here, but insurance documents are not normally signed after a death, unless you are an executor or legal guardian of your sister. I'm not a lawyer, but my experience of being an executor for my own mother's estate makes me concerned about this confusion.
You do not seem to think of yourself as an executor of the estate or as a guardian. It's possible you may simply not have mentioned this, but you may also not have been made aware of some formal legal responsibility you have (e.g. by your stepmother's omission). You really need to get a lawyer to establish and explain to you what your formal legal position is in respect of your sister and your late father's estate.
This makes consulting a lawyer (of your own) a necessity, IMO.
Now it could be that your own behavior, due to depression or anxiety, is frustrating your stepmother and she's trying to get things done in a way that's perhaps not ideal, but getting a lawyer is, at this stage, a good idea as you clearly lack confidence in her actions and while it's not perfect, a lawyer would at least give you some peace of mind to proceed.