Any transfer of assets to you and your siblings would be a disqualifying transfer unless you can prove to the satisfaction of MassHealth that the transfer was solely for purposes other than qualifying for MassHealth benefits. There would be a loss of one day of benefits for every $274 transferred. At this stage, what you can do is pay off their debts, make sure that they have pre-paid funeral and burial plots, and perhaps consider putting funds in a pooled trust account.
I strongly encourage you to meet with an elder law attorney to review the situation in more detail.
Permanent nursing home care is covered by Medicaid, not Medicare, and as noted above, any gifts to deplete their assets now, or any within the 60 months prior to applying, would disqualify them from receiving Medicaid, at least for the period where those asset could have been used to pay for care.
However, that does not mean that there is no way to avoid forfeiting this money. For one, your sister can be reimbursed reasonable rent and costs for sheltering/feeding your parents, and if that exceeds their social security income it will save some of those assets. Also, despite your parents' age, they may be at very different levels of physical and mental health. It is possible that even if there were no assets, one or both might not qualify for nursing home care. If one of your parents is admitted to the nursing home and the other is not, the remaining parent, called the "community spouse," would be able to retain a substantial portion of those funds without disqualifying the other from eligibility.
You should contact a local elder law attorney to discuss your options here. Much more information is needed to determine the best course of action.
Any gift at this point will disqualify your parents from receipt of MassHealth. The disqualification will be calculated at the rate of $274 per day. There may be some crisis planning you may be able to do, but it should be done with the advice of an Elder Law Attorney.
I agree with the above answers. Something else to consider, however, is that if only one of your parents enters into the nursing home the other may be able to protect the excess assets by purchasing an immediate annuity. These anuities have special restrictions, so you should consult an elder law attorney to determine if that is the best path for you.