I am facing a cornelian choice.
Should I use custom social sharing buttons for UI consistency or should I keep them original for better recognition ?
If people don't recognise social sharing buttons they aren't going to use them.
People recognise the logos of their favourite social sharing apps, and you want them to be able to do this as easily as possible so that they are more likely to share them.
If you think that your site looking good is more important than this, then why do you even need them in the first place? Surely it would look better without them messing up your design?
If the functionality provided by the share buttons is ancillary to the main function of your site (as I think they usually are), I would change the icons so that they match the design of your site. After all, it is your site.