Naturally, if you're a co-owner of the property, there's no way your brother can "evict" you. I'm finding it a bit hard to understand why you don't understand you're an owner of the property. :)
Absent an actual mention of "joint tenant[cy]" in the deed, the law deems you two to own as TIC (in other words, if there is no mention of specific percentages in the deed itself or another contract/agreement, 50/50).
Not sure why there was a "lease" as to the home, since you're a co-owner and already had full rights to be there. I gather mom covered the mortgage, and perhaps hers is the only name on it (or you both are on it, but then it wouldn't make sense for you to be paying your mother "rent" each month vs., what I extrapolate, this was something of a "downpayment" arrangement as it relates to buying from her/her estate).
So you figure out what the equity is in the house -- if ANY -- divide that number in half, then subtract whatever the $500/month totaled: that's roughly what you owe the estate for your mom's interest in the place. (All that said, technically speaking, you only need to come up with that number less 25% to represent your interest in it. Otherwise, you'll just get it back from the estate when the estate proceeds are divvied up -- assuming estate debt doesn't eat up everything ... since I gather your mom's is the only name on that mortgage.)
If the estate/probate case isn't closed by the time you want to buy the property, that's okay. Your brother can sign a deed as it relates to your mom's interest *on behalf of the estate*.
You haven't said anything to indicate that your brother's done anything that a court would see as worthy of removing him as PR.
"The other factor in this is that I in fact have a legitimate bases for having my brother removed as PR as a result of the inappropriate things he has done..."
I didn't read every post, but you haven't indicated so far anything in this vein.
The document is gibberish.