Garlic substitution in marinade/paste
I would still suggest using asafoetida (also known as &quot;hing&quot;).  Bloom it in just a couple tsp of vegetable oil,and then add it to the paste.  Since your marinade already has fat in it from the yogurt and buttermilk, a little extra fat from the oil won't change it significantly.  Frying spices in oil separately and adding them to liquid is a common Indian food technique, called a &quot;tarka&quot; or &quot;tadka&quot;.  And asafoetida really is the best garlic substitute ... that's the whole reason it exists, as a food.
In terms of obtaining it, your best bets where you are (your profile says Rocky Mountains) is either to find a grocery store with a substantial Indian food aisle (there are Indian groceries in almost every major Western US and Canadian city), or online ordering.  The nice thing about hing is that a little goes a long way, and it keeps for a long time if it stays dry, so the one little jar you buy should be good for dozens of dinners with your sister.
Substitution ratio: 1/2 tsp is equivalent to 2 cloves garlic, or 2/3 cup minced onion.