The way I do it for myself, assertive is when it is about me, only; aggressive is when I am talking/working on other people's stuff.

Your mother needs to be assertive and tell your uncle to respect what you have to say with regards to her and his business (or, you don't bother with the whole thing, your initial, wise, self-preservation thought :-)

The only way you can be assertive in this case, I believe, is, when cut off, to announce you no longer wish to take part in the transaction in any way and then to get up and leave. One can only speak for one's self, not for anyone else, even though your mother "privately" asked you to. When one demands "rights" (to be listened to) when the situation is not about one''s self, directly, then one is overstepping one's personal bounds and that's "aggression" by definition almost.

Because it is legally about your mother and her brother (or is it brother-in-law?) the only way you can make it about you is if your mother says it is about you and your uncle agrees. Your mother's desire to stay "safe" and have you do her difficult business does not make it your business.