Many life insurance policies are written in trust for named benficiaries so that the money can be paid out soon after death without waiting for probate - in other words, the money is not part of the estate to be disposed of under the will. If your BiL's policy is like this then in effect the money is going to his father without any strings attached. Whether your sister could then claim provision from it (in the same way as she could from the will if that does not provide for her properly) I am not sure.
You say "he thinks his parents will make sure she will get her part" which implies that she should not have all of it. Without knowing the family financial circumstances it is difficult to comment on this, but if he wants the money split there is nothing stopping him getting more than one beneficiary named and specifying the % each is to have.
There can be at least 3 reasons for what he has done:
1. His illness has affected his mind & he is not thinking straight.
2. He has been unduly influenced by his parents.
3. He genuinely thinks his wife would be unable to cope with having the money - eg she has no experience of handling sizeable sums, or she tends to be spendthrift (You will know whether there is anything in this).
Whichever it is, if your sister is going to need the money to maintain the lifestyle of herself and daughter it could be worthwhile making efforts to get him to change his mind, or at least to sign a document (with independent witnesses) specifying the use - for your sister's benefit - that he wanted the money to be put to.