23andme.com will give you ancestry info - maternal and paternal haplogroup - for 99 bucks.
You'd need to be in contact with the sister to find out your uncles results.
So going down that route would lead to communication being opened - which as SS has written - is something you need to decide whether you want to open up to that.
I feel for you - families can be awful! I've had my fair share!
The hardest thing in life is to 'let things go' - we see a situation on the horizon which we know and sense will be stressful - yet we embrace it because we feel out of principle things need to be 'put right' or whatever...
It's very hard to know what to do - and i guess a lot of soul searching is required. If you know none of it bothers you in the grand scheme of things then it's advisable to just not give big sis any attention which she so obviously is desperately seeking by 'stirring' things up.