For the 2 solutions I provided, skip to the bottom of the answer.
For now, male Combee is workable. Since you're in the early game, it doesn't make too much of a difference. However, it will quickly become a pain to have on the team, so it should be discarded probably after the second gym. Gardenia's gym is probably the only place where Combee may prove useful. If you really want to, you can use it up to Maylene, but after that, box it.
Due to the rules of a Soul Link, boxing Combee also would mean your sister boxing Budew. However, this isn't really too much of a problem. Sure, no grass type, but the grass type isn't too important at the point you're at. In Platinum, the major Pokemon that are weak to grass are:
Crasher Wake's Quagsire and Floatzel
Candace's Piloswine
Bertha's entire team except Gliscor
Cynthia's Milotic
Notice how the earliest one you'll find is the 5th gym. It should be noted, however, that Quagsire's only weakness is Grass, so that may pose an issue.
The solution to the grass type issue is simply just to keep trying for a Cherubi on one of the honey trees, or get lucky with a Carnivine or Tropius in the Great Marsh. There are 8/9 honey trees (Not sure if you can go onto route 213 before the gym is defeated) before Pastoria City, and each of those has a 4% chance to have a Cherubi (Correct me if I'm wrong.) This means that there's a roughly 32%/36% chance of getting a Cherubi before Crasher Wake's gym. The Great Marsh is confusing as hell, so I'm not gonna go into the chance of getting a Carnivine/Tropius there, but I'm just saying that it's an option.
Let's say that you've arrived at Crasher Wake's gym and your sister has no grass-types. If all else fails, your sister gets the Budew from the box and evolves it. Granted, you'd have to take a male Combee into the fight with you, but Crasher Wake has 3 Pokemon, and you should have 6.
So, here's the solution so far:
Use Combee and Budew up until Gardenia/Maylene, then box it. With no major use for grass types up until Wake, pray for a Cherubi with the honey trees or a Carnivine/Tropius from the Great Marsh. If those fail, get the Budew and Combee from the box and evolve Budew to use against Wake.
Or...just don't use a Grass type against Wake.
In the event that Budew dies...oh well. Just pray.
For the rest of the game, Cherrim probably won't hack it. You may want to try for a Snover.
However, there's another solution that doesn't involve getting a grass type, assuming you brute force Wake's Quagsire and Bertha's Gastrodon.
Your sister may have gotten a Shinx as an encounter, which deals with the Water types of Sinnoh except for the few Water/Ground types. If she hasn't gotten Shinx, a Pichu/Pikachu can be caught at Trophy Garden, so just pray for an encounter there. If you don't get one there, you can get an Electabuzz/Magnemite later, but by that point you probably won't need them. As for the Rock and Ground types, there are plenty of Water types in the game that can be found by surfing or fishing.
So, two solutions to your sister's problem are:
A) Use Combee and Budew up until Gardenia/Maylene, then box them. With no major use for grass types up until Wake, pray for a Cherubi with the honey trees or a Carnivine/Tropius from the Great Marsh. If those fail, get the Budew and Combee from the box and evolve Budew to use against Wake.
Or
B) Use Combee and Budew up until Gardenia/Maylene, then box them. Use the Shinx line from this point on to deal with Water types (or brute force them until you can get Pi(ka)chu. If you don't, then tough luck. keep brute forcing lmao), and use a Water type to deal with the ground and rock types you find along the way, effectively replacing the offensive properties of the Grass type.
If you don't like that both of these solutions are based on luck, you should not be playing a nuzlocke.
Under no circumstances should you keep the ComDew combo until the end of the game, unless you want to suffer/kill the combo.
If this answer is unsatisfactory, tell me and I'll edit/hide it.
Hope this helped.
