My estate is left to my children, my sisters have signed the will as my witnesses. By law i know that witnesses cannot be named as beneficiaries in my will.
My question is what would happen if myself and my children died together,
my sisters would be my only next of kin. Would they still receive my estate as they are my witnesses even though they are not named beneficiaries in my will.
You must state in the will what is to happen in such circumstances - "if all legatees are dead by the time this will goes to probate, then my estate goes to the Dogs' Home . . . "
It is an interesting question, but I would imagine that in the scenario you describe, your sister would inherit your estate – since she had not benefited from the Will (and was not a named beneficiary of that Will).
Otherwise your sister would have barred herself from her rightful inheritance, simply by witnessing a Will.
If the will is not written to provide for the circumstances you mention, you will be effectively intestate since the terms of the will cannot be carried out. The intestacy rules would then apply. The question of your sisters being witnesses and 'named beneficiaries' to a will would not arise.
In this situation you as eldest are presumed to have died first. The estate goes to the kids under the will. They (presumably) have no wlll and are therefore intestate. It will go to --their-- next of kin in the prescribed pecking order - probably your sisters
The fact they have witnessed --your-- will is irrelevant.