You have a big fat huge mess on your hands. You cannot LEGALLY do anything with this land. Whatever you do is subject to challenge from any other heirs and you run the risk of losing whatever you have invested. If you are okay with that, then go ahead and ranch away.
The only way to legally straighten this out is to get to a lawyer and have him/her sort it out for you. You have at LEAST one probate estate, and perhaps more to deal with. The property is still titled in your grandfather or grandmother's name, most likely, but that is anybody's guess, as well.
If your cousins are in agreement with your plan, then you can buy out their interests. You might or might not get credit for the money that you have "invested" in the land. Your mother's payments may or may not matter. SOME of your cousins would probably be fine with you just HAVING the property. Some, clearly, are not interested in that. There is no way for you to handle this without dealing with them, unless you continue to do what you (and the rest of your family) has always done with this land, which is nothing. Again, that is fine, until someone comes along and objects, at which point, you could have HUGE problems, fast.
Whether that will happen in your case, you are in a far better position to judge than I am. If it were me, I would want to get this settled. I do not know if that is possible, under your facts. You will not be able to do this without a probate attorney, however.
Best of luck to you!
*** LEGAL DISCLAIMER I am licensed to practice law in the State of Michigan and have offices in Wayne and Ingham Counties. My practice is focused in the areas of estate planning and probate administration. I am ethically required to state that the above answer does not create an attorney/client relationship. These responses should be considered general legal education and are intended to provide general information about the question asked. Frequently, the question does not include important facts that, if known, could significantly change the answer. Information provided on this site should not be used as a substitute for competent legal advice from a licensed attorney that practices in your state. The law changes frequently and varies from state to state. If I refer to your state's laws, you should not rely on what I say; I just did a quick Internet search and found something that looked relevant that I hoped you would find helpful. You should verify and confirm any information provided with an attorney licensed in your state.
Do not do a thing until you get the legalities resolved. This is an accident waiting to happen and it has already happended to you. You need to get estate counsel to resolve this mess and ultimately have a partition action brought to bring this to a head. If you do what you contemplate doing you may be throwing away resources. By the way any real estate taxes you pay may not be deductible on your tax return if you are not the legal owner.
Hope this helps.
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In addition to what the others said make sure your lawyers knows estates and also how to handle a "quiet title" action. That may be in the cards. Be sure and mention how you all have paid the taxes.
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