It can be quite expensive to raise a claim of undue influence.  Probate litigators usually charge by the hour and the lowest fee in my area of Florida is $250 per hour.  You can imagine that this adds up quickly - $10,000 is 40 hours at that rate.In addition to attorney's fees, there are fees for expert witnesses such as physicians or psychologists and fees for taking depositions.  Like attorney's fees, these can add up quickly.  Depending on the circumstances, the number of witnesses who must be deposed, the number of expert witnesses needed, and the degree to which your brother-in-law and sister will defend the claim, it could quickly cost $25,000 - $35,000 to bring the claim.Most state laws allow the winner to be paid from trust assets.  The Trustee will take the costs to defend the trust from trust assets, at least at first.  In some states and under some circumstances, both sides will be paid from trust assets.I would encourage you to consult with a couple probate litigation attorneys about your specific circumstances.  Some, like me, are highly offended by undue influence and will work with you to keep the costs as low as possible if you have a solid case.Good luck.