My father-in-law (retired ) wants to move from Las Vegas to Md due to ill health (dementia ). He is a long time Nevada resident but former resident of Md. His adult children live in Md & he needs their help as his disease progresses. He currently owns his own house in Las Vegas but is willing to sell or rent it if need be. He has established a revocable trust in Nevada using Nevada lawyers.
He is well off financially with a net worth of 4-5 million.
What would be the best advice for him if he moves to Md as it relates to :
1) estate taxes
2) state income taxes

Could he establish dual residency

First off, there is no such thing as "dual residency" when it comes to the states. You are considered a resident of one state at a time. Essentially you are a resident of the state in which you spend the most time. It can get tricky for people who move temporarily, but it sounds as if this move will be considered as a permanent relocation for your father. Hence upon moving to MD he will be a MD resident. He will have to start paying MD income tax on his income as of the date he moves. As for estate issues - this forum is not the place to get into detailed discussions - my strong suggestion is talk to an estate planning expert locally who can address these issues properly. But in general - his net worth puts him below the threshold for his estate to have to pay federal estate taxes. But MD's threshold for estate taxes is lower - currently at $1M though rising over the next 5 years to meet the federal exemption by the year 2019. What that means is his estate may have to file and pay MD estate taxes if he passes before 2019. The MD estate tax rate is 16% after the exclusion. But again - with his net worth it is really advisable to get professional advice on possible ways to mitigate that.

I am in complete agreement with ebaines; your father's situation is WAY too complicated to rely on advice from a tax forum for such major decision.

You need to find a reputable estate planner and tax professional (hopefully the same person) to advise you on your father's tax and estate issues.