Having the proper number of witnesses and notarizing the will are part of due execution of a will. But, your question raises a number of other issues beyond a brief summary question and answer. So, I urge you to seek a legal consultation with an attorney experienced in this area.
This answer is provided for informational purposes only. Actual legal advice can only be provided in an office consultation by an attorney licensed in your jurisdiction, with experience in the area of law in which your concern lies.
Contrary to popular belief, a will doesn't have to be witnessed or notarized to be considered valid; the requirements for two witnesses and a notary gives any will that fits those requirements the presumption of validity (the term for that type of verification is "self-proving affidavit"). Additionally, the witnesses don't necessarily have to "know" the maker of the will; they have only to be able to verify that the maker has proven his/her identity to them and that at the time the maker signed the will he/she was mentally competent to make the will.
Any writing signed by someone who clearly states that the writing is intended to be a last will may be accepted as valid, but there may be hoops the person presenting the unverified document will have to jump through to satisfy the Register of Wills that the writing is a valid will.
As to your question about contesting a will, ANY will -- even one with a self-proving affidavit (two witnesses signing with a notarization) -- is open to a contest. The question is, can someone contesting the document succeed? In general, it's not a good idea to have anyone with an interest in the maker's property (before or after death) witness a will, so having non-relatives who are not named as beneficiaries would be preferable to having family members witness the will. Again, having family members or named beneficiaries act as witnesses doesn't mean the will is invalid -- it merely opens it to questions about whether there was undue influence on the person making the will.
Of course, as with all of my online answers, my advice is limited by the brevity of your question and the facts provided. Additional information would be required to provide definitive legal advice, so this answer isn't intended to, and does not, create an attorney-client relationship.