Probate not being executed

[Karlos77]

My sister has been granted probate for my father who left everything to my sister and I in equal share since his wife, our mother, had mental health issues leaving her unable to manage any finances.
A trust was created which contained the house and his main bank account, again with both of us as beneficiaries, and my sister and himself as trustees.
He passed away just over 2 years ago.
6 months later our mother also passed away. I had her pension paid into an account in my name and I managed her finances after my fathers death.
Their house which was only my fathers name was sold by my sister earlier this year.

My sister also applied for, and was granted, probate for my mother and is requesting financial history details of the bank account that was used for her. There's
Considering her hostile behavior up until now (to put it mildly), I would like to see my fathers probate executed first as I do not trust that it will ever be processed and my mothers is an absolute pittance compared to my fathers. I feel reluctant to assist her with my mothers probate while my fathers is still outstanding.
Is there likely to be any good reason why my mothers probate would need to be processed before, or at the same time, as my fathers?

Also, my father has various investments which don't appear to be account for so far. How can I make sure that everything is accounted for for my fathers probate?

Thanks for any advice

[dslippy]

There is no fundamental reason why one (either) should be concluded before the other.

There are well trodden paths in law to ensuring that a probate is progressed. They tend to be expensive. You need to exhaust other means first.