The second part of this question relates to the ability (or not) of a family member to contest a will. As a principle, a person can leave their estate to whoever they wish - but there are some caveats.
There are primarily two tests that may later enable a challenge by your sister:
a) the testator (will-maker) must have had 'testamentary capacity' - this means the person understands what they own, and recognises any individuals to whom they might have responsibilities after death. Your sister wouldn't appear to be able to challenge on this basis, if she is independent.
b) the testator was improperly influenced (when writing the will). This unlikely, but possible.
For these reasons, given that you believe there might be challenge, it would be better to have the will drawn up by a solicitor - if only because it would then be harder for your sister to argue afterwards that one of these events happened - as a solicitor would have evaluated the validity, and be more likely to be able to rebut any statement made by the sister later.