I assume that the nephew is the son of the sister who was part owner of the house?
WARNING - THIS IS NOT LEGAL ADVICE! All I am doing here is preparing you for a visit to a probate attorney, so that you'll already understand some basic concepts when you start discussing your situation. That attorney may disagree with part or all that I'm about to tell you, and he'll undoubtedly be right, but it is important that you have some basic understanding of what you are stepping into before you walk in that door and start paying BIG money, that I want to give you a little heads-up.
When you were given the deeds, what do you mean? Did your sister sign a quit-claim deed on the property, transferring her share to you? Was that deed recorded at the county offices? What about your mother? Did she quit-claim the property to you as well? If so, you may be OK. How was the initial (and subsequent) deed written and recorded? You'll need copies of the recorded deeds and any supporting paperwork to bring with you to the attorney.
If the property was in your sister's and mother's names only, then unless they specified otherwise, as joint owners they each owned 50%. The way that the ownership was written "Jill Jones AND Jane Jones" or "Jill Jones OR Jane Jones" also makes a difference. *Unless your name was on the deed, you have no rights to the home* When your sister died, any transfer of full title to your mother should have been accompanied with a trip to the county and filing of the certificate of death. If that was properly done, your nephew may not have a claim. When your mother died, the property should have been distributed, according to the following basic priorities (in order of legal precedence):
1. According to any revised ownership as shown on the recorded deed. If you are Jeanne Jones, and the deed at your mother's death was made to "Jeanne Jones OR Jane Jones," and you brought your mother's death certificate to the county, then the house would be 100% yours.
2. According to your mother's will, assuming that she at that point was 100% owner.
3. According to your mother's and sister's wills , assuming that they were both still listed as co-owners.
3. If neither of them had wills, and both were still listed as owners, then they died intestate, and the probate laws would apply. A copy of them is here:
ESTATES IN ALABAMA
From this point lower on the list of priorities on how the property should be distributed, it gets much more complicated.
One scenario is that the offspring of your sister should have shared equally in her half of the house upon her death, and when your mother died, your mother's offspring should have shared equally in the other half of the house. The fact that you paid the payments, or took care of them, is pretty much legally irrelevant. By that scenario, your nephew can lay claim to a percentage of the house, but not all of it.
What you have depended upon is the honor and trust of siblings and offspring, rather than doing the legal homework to make things by-the-book. That hasn't happened. At this point, the probate process to clear things up will be expensive and contentious. There may be some claim you have of adverse possession, but I doubt it holds water.
I suspect that the ownership is clouded and that you will have to go through a process where your siblings, and any offspring of your dead sister provide you with quit-claim statements, and that the nephew (who would only own a percentage of the house in just about any scenario) will have to be bought off, or a forced partition of the property by sale of the house occur to satisfy his claim.
Read the above through a few times until you have the concepts down, then gather your papers and copies of documents from the mortgage company and county, and head down to a probate attorney. You can expect the process of clearing the title to be both time consuming and expensive. Good luck.