Hajj & Umrah

Woman refusing to go for umrah with her brother

Q: My mother in law has been asked by her brothers to go for Umrah with them (they will pay all the expenses) but she refuses saying that she will go for Umrah only with her sons (they dont have enough money at this point in their lives). She says that she doesn't want any favours.

The question is, is she allowed to refuse Umrah? Would it be a sin? Or can she wait for her sons to take her to Makkah.