"My father died this past November and left everything to myself and sister, including his house/property. If we sell this property this summer, do we have to claim a capital gains? If so, on what part?"
On the transfer of the property from you to a third party, you are probably going to be liable for capital gains tax. The period that you're on the hook for is from the date you acquire it to the day you sell it. Since this period of time will be only a matter of months, the property might not incur too much of a gain in that time, and therefore your tax will be small. You and your sister can split the tax between you as you are both inheriting the property.
Note that if the property in question is your principal residence (which doesn't seem to be the case here), the tax is not payable on your sale of it, because this is an exception to the general rule of capital gains.
Now let's look at the first transfer - that of the property from your father to you. Since you describe it as "house/property" rather than just "house", I'm assuming there is something more than just a house. When these items transfer to you, there is no tax for you personally to pay. However, there certainly may be taxes that must be paid by the estate.
When your father passed away, there would have been no capital gains tax payable on his home (principal residence). Note that only 3 acres of property can be included in the principal residence exemption. If there was an additional property, such as a cottage or revenue property, there is capital gains tax payable on that property. Keep in mind that if the property sits in the name of the estate for a long time, there may also be tax payable on the increase in value while it's in the estate name.
As you can see, it seems a simple question but the answer is complicated. This is why I recommend that you sit down with an accountant to figure out the tax details.