There may be ways to access the retirement money earlier without the penalty, so keep that in mind.
For the question at the heart of your post, it's a somewhat heated topic, and generally when it comes up around here, the consensus is "never count on an inheritance". But I disagree somewhat with that. You need to carefully evaluate your situation. Is the family dynamic such that it's almost unthinkable you'd be disinherited? Have the parents explicitly told you you will inherit everything (as opposed to giving some or all to charity, for example)? Are you privy enough to their finances to know roughly what the amounts might be and whether there is any reasonable chance they could spend much of that down (long term care required, for example)? Do they have good health insurance? LTC insurance? (Not a requirement, but one factor to consider when evaluating how certain the inheritance is.)
If you've looked at those things carefully, personally, I don't see anything wrong with keeping in mind an inheritance. My parents have been very clear with my sibling and me on their will and their estate. They have very solid medical insurance. In their 70s, they still have positive cash flow (2 pensions, some consulting work, and a few other things). Our relationships are rock solid. They've made it very clear that they very much want to leave generous sums to Sibling and me, and in fact have a goal number in mind that they've shared with us, so it's clearly important to them to do this, meaning they are going to try damn hard to make sure it happens. Like you, I wish they would spend a bit more, but they are set in both their ways, and their desire to do this for their children. I still don't take a specific number and insert it into our FIRE plans, but I do mentally consider it a cushion that will allow us to be less conservative about our plans. We might push for a lower SWR, or a budget number with more wiggle room if that money wasn't in the back of our minds, so I guess in that sense, we have adjusted our FIRE plans based on this. And again, my parents have encouraged this mindset, so I don't think it's greedy or presumptuous. They've explicitly said they want to give us the ability to quite work earlier and/or travel more (our passion).