Don't study languages that are too closely related. It's easy to learn them passively, but when you start using them actively they turn to mush and it can take a lot of hard work to separate them. My Russian and Serbian got badly mixed for a while.
Once you know one of a closely related set of languages well for active use, it's safer to branch out. My Spanish was okay, French wasn't too close, but Italian and Portuguese don't affect my Spanish. If anything, the stronger language affects the weaker ones (and will take some time and effort to sort out).
Same for alphabets. Iranian and Arabic, not closely related (unless you like time depths for things like Nostratic). Still, you don't want to learn two entirely sets of values for the symbols if you're learning the alphabet for the first time.
The only real issue is time: Do you have the time to work at them equally, or do you have different expectations for learning times? Are they languages that are roughly equally hard to learn, or is there a built-in differential?
Even if you're able to keep them straight most of the time, every once in a while it can be overwhelming. When I was studying simultaneous interpretation for Spanish/English while taking an advanced Russian conversation course and a couple of literature courses taught it Russian my Spanish and Russian would sometimes collapse into a heap--it wasn't like mornings were in one language and afternoons in another. No, they alternated and by dinner time I had to just sit in silence and let the babble in my brain settle down. I'd stick Russian verb endings or prefixes onto Spanish verbs, or vice-versa. Word order was scrambled in Spanish from time to time. And let's not even talk about Russian vowel reduction in Spanish or Spanish stress patterns in Russian. Just ... no.
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