Similarly, you can also change the collisions for a quick fix; same thing, select the bikini, then in the parameters tab change the collision object from your character to her boobies. This helps with little things where changing the whole fit isn't necessary, or impractical due to it clipping with the character now.
When messing with characters like we are doing, cutting out parts of the mesh and slapping on new body parts, we're making the clothing try to do stuff it wasn't designed for. There are fixes that are quick and easy, but these are just kinda like bandaids; help staunch the bleeding, but still leaves the user quite in need of healing (or fixing things in our case) on their own. The actual way to fix this would be to probably go into the bikini itself and make morphs for it to follow Breastacular, JCMs or whatever their equivalent would be in the clothing/prop terminology.
I'm not smart or good enough to do that stuff, so I play with what's available in the Studio program itself, like these quick easy bandaids.
Another option (also out of the ordinary for what I would do, but have I the past for somethings) is to make two renders and superimpose one over the other in post and blend them together, taking the breasts or whatever from the one render and placing it over the other render with the correct pose.
Not something I would want to do a lot, but a tool that you can use from time to time.
Just keep in mind that this is, again, not what the figures were designed for, so clothing makers can't really make things work with every after-market body part a user might slap on their character. Something like this isn't an issue with Breatacular but something that needs to be fixed on the clothing side, because the clothing doesn't know it's covering the Breastacular graft. As far as the bikini top is concerned the graft doesn't exist.