It is not polite to shove one person aside in order to have a conversation with another that you don’t include in the conversation, but give her some room.
First off, you do not know that this other friend is not in some particular need that your godmother isn’t at liberty to tell you about. Besides, it is your godmother’s prerogative to treat this lady like another sister, if she wants to. Do not be jealous because she is kind to someone other than you. For all you know, this friend is also helping her in her bereavement or helped her when your godfather faced his death in ways that you do know nothing about. It may be that she helped your godmother in her time of deepest need, and now your godmother is returning the favor because the nurse is going through a difficult time herself. Do not presume that this nurse does not deserve the special place she has with your godmother, then, whatever you do. That is not for you to judge, not even if you were your godmother’s blood daughter.
You do have room to ask if there is a rift that you do not know about. When there is a rift like that, it is gracious (and wise) to ask if you have done something to offend the person who seems to have cooled to you, rather than jumping in and telling her that she has offended her. Heaven forbid that you have unintentionally hurt your godmother by what you have done or failed to do, and then attack her because her feelings were hurt by your thoughtlessness. So when you bring this up, open the conversation by asking if you have done something to offend her.
When you have done that, really listen to what she has to say. Be open to what she has to say. Do not judge what she has to say as if you have a demand to make on her and she has to have an excuse not to do as you want her to do or if she is required to carry on after her husband’s death just as she did before. Not only has she suffered a great loss, but there is also a limit to what a godmother has a duty to do for her goddaughter. What you have had with her is mostly from the goodness of your godparents’ hearts, not something you ever had a right to. Do not presume that this goodness is something you are entitled to, OK? You are used to it, you have returned it in kind, but you are not entitled to it. It is something you have been giving each other by mutual kindness, so keep the relationship on that level.
If you have someone else you can talk to about this, someone you know will keep what you have to say in confidence, that wouldn’t be a bad idea.