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First, I am sorry for your loss.
The only reason to file the Will is if there needs to be a probate and if the "assets in your mom's name alone are under $100,000" there is no need for a probate.
Assets that are in joint tenancy have beneficiary designations (like life insurance) do not count toward the $100,000. In other words, there could be $1 milllion in assets all in a joint account or life insurance policy, or a combination of both, and still no probate would be needed. So your brother may be correct.
Still, unless he is hiding something, he should provide you an accounting of everything. If he doesn't, and you want to force him to do so, you will need to get the court involved and for that, you will likely need an attorney (although it's not required to have an attorney). Keep all the written/typed/emailed requests as well as all notes of your conversations for evidence in case it is needed.
Only if there is a probate do you get the opportunity to approve of the executor. With small estates (under $100,000), there is no such need. And if it is a small estate, he is required to wait 40 days before using a small estate affidavit (under Probate Code 13100) to obtain the assets in your mom's name alone.
Finally, there is no time limit to file the Will -- if a probate is needed. It is up to the named executor. But again, if he is delaying, you as an interested party can petition the court for an Order to Show Cause (OSC) as to why exactly he is delaying matters. Thank you.
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