It is correct to have placed the question under the French Law / European Law category.
The legal situation which you have described in your question is that you, together with your mother and your two sisters are co-owners of the house. In Civil Law systems, such as France, every co-owner of an immovable property, such as a house, has some rights, regardless of their share of that property. These rights include the right to decide (in common with the others) on the use of the house; and the right to be "taken out of the co-ownership".
The right to decide how to use the house means that if the co-owners do not agree together on how to use the house, only the French court may decide, based on the competing proposals, how the house should be used. It might allow you to live there, in exchange for a rent based on their share of the house, but it might also adopt their counter-proposal, if there is any. Moving in without their consent might give them cause to sue you for unauthorized use of the property (i.e. civil monetary damages in lieu of rent and expenses).
The right to be "taken out of the co-ownership" means that any co-owner may choose to realize their share of the house. This means in practice selling it. Because they only own a share, they cannot sell the entire house, just the "ideal" (= legal, percentage, on paper) share which they own. If they sell the ideal share, somebody else will take their place in the co-ownership with the same rights.
The other option is to force the "splitting" of the house. This can be done in kind (i.e. if they wish to live at the same time with you there, they can ask the court to allocate rooms), which however is very odd and usually does not work except for very big properties. It can also be done through a forced sale. However, usually the other co-owners have the right to purchase the shares at market-value, or if you cannot agree on one, at the value set by the French court.
To answer your third question, as long as both you and your mother live, there is nothing to leave to your wife except your current share. When your mother will pass away, if you live at that point, you may inherit a part of her share, which will be added to your future estate (of which you may wish to dispose in a Will).
In terms of practical advice, if you cannot reach an agreement with your sisters, I would suggest taking a consultation from a French attorney localized where your house is, because the local court would be competent for matters such as usage rights and forced sales. S/he would be able to tell you what the procedure and costs (and chances) are for obtaining the right to live in the house. If you do agree with your sisters, the prudent thing to do is visit a local French notary and record the agreement in an authentic document, so that no questions arise where someone changes their mind later.
Also, regarding your Will, you might not have heard it, but on 17th August Regulation 650/2012 entered into force. While it does not apply in England, it does apply in France, and you may wish to consult a (French or English) estate attorney before making your Will, so that it is valid in both countries.
I hope my answer clarified your options and you will be so kind as to offer a rating.
Dr. Ioan-Luca Vlad