Dear Jackie,  

If religious life is for you, you can always tell your mother that the best Spouse possible has proposed to you.  

There is no reason for her to lay the guilt or burden on you of fulfilling her dreams, when your sisters should bear their share. You don't say why they don't want to get married and have kids, but I am presuming it is not because they want to be nuns too.  

It is very easy to understand your mother, however, so you should be very sensitive (you seem to be, but excuse me insisting) to what it must mean to be separated from her husband, especially if she loves family. Try to help her grow in her spiritual life, if possible pray together, go to Mass together, and open the way to deeper conversations about life and what God may wnat you to do.  

God bless.