Q: In "Jack will come to mother's house and will come to father's house so long as X", does "so long as…" apply to both preceding conditions? I need to interpret the following sentence:

With regard to Christmas this year, Jack will come to mother's house and will come to father's house so long as Jill does not have Covid.

Does the "so long as Jill does not have Covid" apply both to Jack going to mother's and Jack going to father's house?

A: We know nothing from the sentence about the relationship between Jack, Jill, father or mother. For all we know, father or mother may be the writer's parents rather than those of Jack or Jill. They may be living together, or apart.
As it stands, the sentence is unclear because it is not clear to what the conditional part "so long ..." applies:
Jack will come to mother's house and will come to father's house so long as Jill does not have Covid.
Let's insert a comma:
Jack will come to mother's house and will come to father's house, so long as Jill does not have Covid
Now it is clear that the part before the comma is a unified proposition formed by the conjunction "and". It will only satisfied if Jill does not have Covid. In that case, Jack will visit father and mother.
Now try the comma in another place:
Jack will come to mother's house, and will come to father's house so long as Jill does not have Covid
Now Jack will visit mother. But he will only visit father if Jill does not have Covid.

A: 
With regard to Christmas this year, Jack will come to mother's house and will come to father's house so long as Jill does not have Covid.

If Jill is Jack's wife, then it applies to both.
If Jill is his father's wife, it applies only to the latter.

A: 
With regard to Christmas this year, Jack will come to mother's house and will come to father's house so long as Jill does not have Covid.

The sentence is ambiguous, though context is likely to indicate which interpretation is correct.
To see why this ambiguity exists, consider two examples:


*

*With regard to Christmas this year, Jack will come to mother's house and will come to father's house so long as Jack is able to travel.

*With regard to Christmas this year, Jack will come to mother's house and will come to father's house so long as father promises to be nice to him.

The sentences share the same basic structure (up to "as"), but sentence 1 implies that the "so long as..." clause qualifies both instances of "will come," whereas sentence 2 implies that the clause only qualifies the second instance.
Therefore, we can conclude that such sentences can be interpreted either way, though the content of the "so long as" clause and the surrounding context can disambiguate between the two.
