I personally don't believe it's your responsibility to care for someone who didn't raise you. I think you're right in saying it's not really even possible.
As to the phone calls, can you put a block on their numbers? How many times have they called? Do you have records? If it's to the level of harassment, you can report it. You can warn them you will report if they keep calling. You can block their phone numbers and e-mail addresses.
Last time my dad called (nearly 4 years ago) and left a dozen voicemails swearing at me, threatening to show up at my work, calling me names, and accusing me of one thing after another, I blocked his number. Problem solved.