Hmmm....

A few things can be said.

This is your parents' decision. Period. What you and your siblings think doesn't matter, even if your parents' decision is objectively stupid. If they want to donate all their money into making Wal-Mart better, then you have to deal with it, you know?

You say "none of us have a problem with this." Wrong. Your wife has a problem with this. This decision affects you, and thus it effects her. The fact that you and your brothers are okay with it doesn't mean "none of us have a problem with this", because you have to consider your wife, even if your parents aren't leaving her anything in a will.

I would suggest talking to your brothers (and sister) about a "kids fund". Rather than leaving the eldest brother more money because he has kids, see if your parents can leave each brother the same amount, and a separate fund for "any grandkids". If nothing changes, your oldest brother's kids would still get it all, but if you or your middle brother or your sister had kids, they wouldn't be left out. I think your wife's base complaint here makes sense, though she expressed it like an angry child ("I guess we should just have kids then" is a scary/dangerous threat, even if she didn't "really" mean it, you know?) It's an understandable complaint that could be expressed and handled as an adult, and amidst the main problem here, I figured I'd mention it because it could make everyone happier in the long run.

Your sister didn't make this decision. Your wife being angry with her is Strike One. It's an irrational lashing out at someone who doesn't deserve it for something that isn't her decision to begin with.

Your wife being racist about it is Strike Two. Anger is no excuse. Especially when it's not something where the anger is justifiable in the first place. (if you do something bad "because of anger" but the anger is justified, it's still "one strike", so to speak, you know?)

Your wife not apologizing is Strike Three.

Remember how the friend handled it. If shit goes to hell, remember not to direct any of it at the friend.

Your wife needs to apologize, to you, and you need to be confident that it was a one time thing. Maybe some form of counseling? Since she didn't have a very adult response to bad news, and followed it up with a not very adult response to being called out on it. She's 25 and married, and needs to be beyond that. If she won't/can't, then it sucks for you, but you need to reconsider if you want to be married to a child with a poor opinion of your family/sister.

There are many scenarios where being upset about a will would make sense, but I don't think this is one of them. The tiny legitimate concern about kids is ruined by the hostile reaction towards your sister (who again, is not at fault in any way). I only mentioned a solution because you might find someone worth having kids with where you'd want to do everything you can for them. (since people generally don't make the drastic changes your wife would have to make before people would be willing suggest it's a good idea to have kids with her)