If I already have Umbrella insurance; what would I need to change wrt my Umbrella insurance to get a room-mate or a tenant in the in-law unit in my backyard? 

In addition to the trust, they will simply protect their assets by getting an umbrella insurance policy.

I have an umbrella policy already in place on my end. 

My recommendation would be to go with either an LLC or even a Sole Proprietorship, and buy an umbrella policy to protect your assets over and above what is covered by your normal landlord homeowner's policy.

Do you mean an umbrella policy for that property?

Take a look into an umbrella insurance policy.

Our family team has agreed to pursue the umbrella policy of 1m covering all personal property.

One alternative that was made available to me was just setting up a partnership with both my mother and my uncle and have umbrella insurance covering everything, but I am stuck on the legal aspect of this should any of the tenants chose to sue us down the road. 

I have been working with a partner to purchase multiple SFRs through an LLC which we co-own, to be financed under a single commercial umbrella loan.Basic structure being a 90/10 split on initial cash contributions; I manage every aspect of the property from aquisition, to improvements, to tenant managment; and then we have a 50/50 split of profits.We currently have 1 property under contract, but are struggling to get the 4 we were hoping for.

Then just get yourself a nice umbrella policy just in case.