Is the first input in a transaction always the sender's? I'm trying to figure out what the input address is from a transaction with multiple inputs. Can it safely be said that the first input in the inputs of the transaction is the sender's?
Can it safely be said that the first input in the inputs of the transaction is the sender's?

No, no such ordering exists.
Transactions consume UTXOs as inputs, and create new UTXOs as outputs. A transaction can have multiple inputs from one person, or inputs from multiple people. In some cases, more than one person can own an input. So there isn't really any useful way to point to a particular input in a random transaction, and say &quot;this is from the sender&quot;.
In cases where there are two or more inputs, there is also the potential that the transaction is a 'payjoin' style transaction, which is specifically built to break common heuristics that attempt to determine UTXO ownership/transaction participation.
More generally, it is prudent to craft transactions in ways which do not surrender information needlessly, so that you can maintain privacy while still engaging with the Bitcoin network.