[Just to be pedantic about it] -> Technically Speaking… Princess Young Heart & Tommy are not “cousins”, but rather she is his aunt.
-> Applebloom is BigMac´s sister.
-> BigMac is a component of Tommy´s adoptive father (King Speedy Hooves)
The sister of a father is the aunt of his son.
Naw she just a super light alicorn filly that aint the chonk ponies you see otherwise. Also yes he is a human so thats a factor.
Edited
Interesting thing to point out…
Good job
-> Applebloom is BigMac´s sister.
-> BigMac is a component of Tommy´s adoptive father (King Speedy Hooves)
The sister of a father is the aunt of his son.