There are multiple ways of addressing what you want. However, the best solution will depend on a lot of factors:

1. The distance between the farthest parts of the two house where WiFi coverage is required
2. The number of floors each houses have
3. The kind of materials the houses are mainly built with
4. How bad is WiFi interference in the 2.4 and 5GHz bands within the vicinity

Offhand, here are your choices:

1. The SIMPLEST and cheapest approach is to reposition the lone AP/Router so that both houses can get sufficient WiFi coverage.
2. Another approach is to utilize two (2) APs. The first one would be your AP/Router servicing your home. The 2nd one would be situated by a window facing the 2nd house, preferably on a higher floor, using either the built-in omni antenna or an external sector antenna.
3. The IDEAL method is to have two (2) APs and two (2) wireless bridges. The first AP would be your AP/Router in the first house while the 2nd AP would be in your relatives' house. These two APs will be joined by a pair of wireless bridges (search in Google for a description).

Use different channels for all APs. The two bridges should be using the same channel but different from the two APs.

Example: (can be in any combination of the three non-overlapping channels: 1, 6, and 11)

AP1: Channel 1
Bridge 1 and 2: Channel 6
AP2: Channel 11