You don't need a solicitor to prepare a will. Neither do you need one to act as executor. (Indeed, I'd definitely advise against appointing a solicitor as an executor).
You only need a single executor. (It's common practice to name two but it's not a legal requirement).
What is vitally important though is to ensure that there are two witnesses to the testator's signature. Those witnesses can include an executor (or be both executors, if two are appointed) but they must NOT be beneficiaries under the will (otherwise they will lose their inheritance). The witnesses MUST ACTUALLY SEE THE TESTATOR SIGN his/her will. (The witnesses don't need to see the contents of the will, or even to know that is a will; they're simply affirming that they saw the testator place his signature on the document). It is NOT good enough to get the testator to sign and then to get people to add their signatures as (alleged) witnesses later on. They MUST see him/her sign.
However the actual choice of witnesses is unimportant. I signed my will on a Tesco car park and had my signature witnessed there by a couple of near-strangers whom I happened to be working with that day!
Assuming that your brother has already passed away when you prepare your will (or that you see no point in leaving him anything anyway, as he will almost certainly pre-decease you), the following is all you need:
"WILL of [your full name] of [your address]
1. I revoke all previous wills and codicils".
2. I appoint my sister [your sister's full name] of [your sister's address] to be the sole executor of my will and I leave to her all my property of whatever description.
Date: [date of you signing the will]
Signature: [your signature]
Signed by [your name] in our presence and then by us in hers: [signatures of two witnesses, who must NOT include your sister]"
That's it; a complete and valid will. A few points to note though:
The bit about revoking all previous wills and codicils should be included even if you've not previously written any wills. [It saves the executor from having to hunt around for any earlier will that might still have some validity, even though no such will ever existed].
Ideally, your will should state what should happen to your estate if your sister was to die before you do. So it would be wisest to add Clause 3, along the following lines:
"3. In the event that my sister [your sister's name] shall die before me I give all of my property to [your choice of beneficiary]"
You could also choose to state who should become the executor of your will if your sister dies before you but it's not essential to do so. (If you didn't, your will would still be valid but whoever was to benefit under Clause 3 would have to apply for Letters of Administration).
Lastly, it would be best (but not essential) for the witnesses to your signature to print their names and addresses under their own signatures.