You don't say where you are, so you (or more properly your parents) need to take qualified legal advice for your jurisdiction.
That said, and with due caution about taking advice over the internet, I would say -
1. Your parents have old wills that they need to update. They each need to make a separate will with provisions that deal with the a) will-maker being survived by their spouse and b) the will-maker being the survivor. If inheritance tax is an issue, it is usual to put in a provision which deals with both spouses dying within a short time of each other (such as a car accident in which one dies immediately and one shortly after, in order to avoid the estate being taxed twice).
2. Your parents need to settle on an executor they trust to carry out their wishes. A sibling they have issues with is not a good bet. Their only child probably is. I'm guessing their previous will was made while you were still a minor, in which case one of the updates they need to make is to make you the executor.
3. If your parents want you to get the money and if you die first for your children to get the money, then the formulation will be something like "to my son X and if he predeceases me then to his surviving children (including children in utero) in equal shares, his wife Y to be trustee if they are underage".
4. The "per stirpes" thing is about making sure the second generation down gets its share. It isn't relevant at the moment because you are an only child (ie you have no dead siblings with children who should inherit their dead parent's share equally between them - which is what "per stirpes" means) and you don't yet have kids who are old enough to have their own kids. They could put it in now to protect your potential grandkids, I suppose, if they wanted to future-proof for a couple of decades.
5. If you die without having kids, then the choices are either that the will makes specific provision for what happens (eg "all to the cat's home") or anything which is not made provision for, including anything left to you, will be distributed according to the laws applying in the jurisdiction they were living in when they died. Typically for someone dying without spouse or kids that could mean equally between siblings and if siblings are dead the sibling's share equally to their kids (the per stirpes thing again). The only way to avoid this is to make specific provision in the will.
6. Murderers can't benefit from their crime, so a poisonous wife, if convicted, would not inherit. Better to choose your wife carefully and build a good relationship with her than rely on this legal safeguard.
7. Trusts are used when you want a person to have the benefit of the money but you trust the person getting the benefit a lot less than the person you appoint as trustee. They are usually compulsory for gifts to children who are underage. Trusts can be expensive, inefficient, inconvenient and the cause of bitter dispute. On the whole, and unless there are specific known problems (eg drug abuse), better to express an informal wish and hope for the best. As long as you are a normal responsible person there is no need for your parents to create a trust to force you to only spend the money on your non-existent kids' college education.
8. You should make your own will too, even if it just says "everything to my parents" it will simplify things for them at a difficult time. It would probably become ineffective if you marry.
9. It would be helpful for your parents to make sure that records of all their assets are kept in a way which their executor will be able to easily access and understand, so that nothing gets "lost". Otherwise nothing special needs to be done with regard to the way in which the assets themselves are held, as this can all be dealt with at the time.
