It should be understood that the inheritance share is a mandatory right prescribed by Shariah, which becomes the property of the heir, regardless of whether they want to own it or not. Therefore, in the situation described, if the sisters merely state verbally that they forgo their inheritance share, it does not relieve them of this right.
However, once the estate has been divided and the sisters have taken possession of their share, they are permitted, according to Shariah, to gift their portion to another heir or any other person if they choose to do so.
It is important to note that coercing an heir into relinquishing their inheritance share due to family or societal pressure, as is often the case in certain communities where sisters are coerced into forgoing their share so that their brothers can receive a larger portion, is not permissible according to Shariah. In such cases, even if the sisters verbally forgo their share, it still belongs to them. If the brothers take possession of it, it is considered haram and an act of oppression according to Shariah.