Layer2 between two locations that are otherwise independent of each other is almost always a bad idea.
Go for a routed connection instead (use GRE tunnel interfaces, put a /30 of private IP addressing on the link, and route each site's LAN addresses across the tunnel.
If you use 192.168.1.X/24 at your house and your sister's house uses 192.168.2.X/24, here's a quick example of what to do:
/ip address add address=192.168.255.1/30 interface=gre1
/ip route add dst=192.168.2.0/24 gateway=192.168.255.2
/ip address add address=192.168.255.2/30 interface=gre1
/ip route add dst=192.168.1.0/24 gateway=192.168.255.1
Then make sure the NAT rules won't apply to packets going through the tunnel interfaces, and also make sure that the FILTER rules permit packets in/out of the tunnel interfaces, and you'll be set.
The #1 reason layer 2 is bad is that both sides will have their own DHCP server, and these will fight each other for supremacy. (Like Highlander, there can be only one) - If you must use layer 2, then perhaps only the device requiring layer2 can be bridged, while the rest of your sister's LAN can be its own network.
When given a spoon,
you should not cling to your fork.
The soup will get cold.