As noted by one of the other answers, a life estate does not require a person to occupy that property - it simply means the owner has all the rights of ownership during their lifetime. Once the grantor dies, the transfer to you and your siblings becomes complete. Your retired sister has no right to unilaterally decide she and her husband will occupy the property. If your grantor is now incapacitated (living in a nursing home) it may be prudent to look into having one of the siblings appointed as a conservator ot handle the grantor's financial affairs. Your retired sister may be liable for rent for her occupancy of the cottage; taxes are the responsibility of the grantor while they are alive, so if you have paid any of those, you may be entitled to compensation when the cottage is ultimately sold. At some point it seems the place will have to be sold, as you and your siblings have not been able to come to any agreement. You also want to consider Medicare implications if the place were sold while the grantor is still alve.
What you have is a mess. You can see how this is all unravelling, already. You should try to work things out now, or you are likely to have worse problems, down the road. Perhaps the siblings that do not want to pay the expenses can have their share bought out by the other siblings.
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None of you have any right to occupy the premises. You could become the conservator for the life estate holder and file an ejectment action against your sister on the life estate holder's behalf. Of course you would have to make sure the taxes and other expenses are paid out of the ward's estate, otherwise the property would be lost to unpaid taxes. Perhaps that is why you have the arrangement you have set up with your siblings in the first place - the ward is in a nursing home and has no resources to pay these things. If you cannot get along with your siblings, you could always file a lawsuit and pay attorneys to reach the same kind of settlement you could have done on your own if you and your siblings could get along. The fair thing is that the expenses should be divided based on the amount of use each gets out of the property. If the sister is living there year round, she should be paying the taxes, etc. If you get to share with her for two weeks, then you should pay half of those costs for the two week period.